Review Apple Fusion Drive: Rehabilitation of “hybrids”

Review Apple Fusion Drive: Rehabilitation of “hybrids”

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    Contrary to expectations, which led to the emergence of the first SSD in consumer devices, is now quite evident that a total transition to SSDs is not possible in the foreseeable future. Manufacturers of NAND Flash-memory made great strides in reducing the cost of the chips, but the price of a gigabyte-capacity magnetic disk is still an order of magnitude lower than in the SSD.

    In addition, with each step Flash-memory to a “thin” productive rate harder to ensure a reliable reading of the charge in the cell and the required number of write cycles. In hard disks, on the contrary, there is still an inexhaustible reserve of capacity expansion in the standard design with familiar GPP / GMR-heads (Perpendicular to Plane / Giant Magnetoresistance) and several exotic technologies in the future.

    The most widely used Flash-memory in mobile devices, where the benefits of SSDs fully justify a higher price per unit volume. If we talk about Apple, then as a producer certainly expensive technology it can be installed in computers SSD, in capacity competing with hard disks that we see in the top configuration MacBook Air and MacBook Pro with Retina Display.

    A unique mobile Apple, which still comes in configurations with a hard drive – it obsolescent each year MacBook Pro without the “retina.”

    In desktop computers, even for Apple is not so easy to drop the hard disk drive as the base case. If your Mac Pro (fully translated into SSD) extends the principle of economy to a limited extent, the three basic models of iMac, which now produces Apple, are composed of a hard disk capacity of 500GB – 1TB.

    There are options with fully solid state drive capacity of up to 256 or 512 GB, which require an additional investment of US $ 200-500 over the configuration with terabyte HDD. And between these two extremes – Fusion Drive, which is an array of 128 GB SSD and HDD to 1 or 3 TB. Upgrade “Mack” on Fusion Drive in conjunction with terabyte drives also costs $ 200, so that the difficult choice. On it will be discussed.

    To be honest, the author of this article is no longer expect anything good from hybrid drives. While none of the tested solutions Gadget Help not come close to producing the declared goal – to combine the performance of SSD and HDD volume with an affordable price.

    Most of the “hybrids” are built on the same scheme: a solid-state component is used as a small amount of cash, which is duplicated frequently requested information from the primary drive – HDD. In some cases, SSD is also used for caching the record.

    It is assumed that after a period of adaptation of the cache begins to duplicate most driving data – OS files and applications, and the debris is rarely sought after resources are the HDD. In this concept, of course, that the more solid components of the hybrid drive, the higher average throughput.

    For example, 8 GB Flash-memory soldered on disks Seagate SSHD, obviously not enough for the system to be as responsive as a fully solid state storage. At best we can talk about this kind of intermediate position “hybrids” between SSD and HDD, although in numerical terms – not even on average.

    A more effective approach is implemented in the technology Intel Smart Response, which allows you to use as a separate cache SSD SSD capacity up to 64 GB (assuming that the system is built on a certain chipsets Intel).

    But at the same time increasing the price, and for this reason a few of finished computers and laptops enjoy such luxuries. Finally, for sufficiently large SSD has a thought: but should not the cache and refuse to take him completely under the OS and programs as a separate volume?

    Actually, it came with a disk WD Black2, which is a terabyte HDD and SSD separate volume 120 GB in a single package. But again, saving on components SSD, multiplied by the raw software, and does not allow WD Black2 lead as an example of successful hybrid drive.

    Meet the Fusion Drive

    So, what can add to this Apple? Recipe Fusion Drive in general the same as that of a Jew from a joke: “Most of the welding place.” In the role of a solid-state component of the array in the “Macs” are the SSD of 128 GB, and that is important – with quality controllers.

    In principle, this is in itself a satisfactory configuration for comfortable operation. By following certain discipline can the OS and all the running files keep within the scope of 128GB SSD (ask users MacBook Air), and library resources and other heavy manual stored on the hard disk.

    However Fusion Drive out and Fusion, SSD and the HDD that are combined into an array. In this case, the end result depends on how the SSD is used.

    Apple iMac 21.5″ Mid 2014

    The total volume of the array is equal to the volume of the individual components. We tested the iMac with a diagonal of 21.5 inches, and in our case it is 1128 decimal GB (1 TB HDD plus 128GB SSD).

    That is, we can just say that the data is not duplicated, SSD is not the function of the cache. Instead, there is a separation (tiering): frequently requested data arrives on SSD, unsold – the HDD. Q: how to identify priorities? But more about that later.

    Fusion Drive: components

    First you need to understand how we got to the iron. In models of iMac and Mac mini to 2013 were used variations of Samsung PM830 – not the worst drive to the SATA 6 Gb / s. Apple now universally implemented with the native interface SSD PCIe, which promises a considerable increase in speed.

    Hard drive – 1 TB capacity in a form factor of 2.5 inches for the younger iMac and Mac mini or 1-3 TB in the form factor of 3.5 inches for the 27-inch iMac.

    System Information utility displays manufacturer SSD. Judging by the name APPLE SSD SD0128F, it is the same drive SanDisk, which can be found in the Mac mini, and MacBook Air and MacBook Pro with Retina Display of the sample in 2014.

    It is possible that an alternative that delivers Samsung, may also occur in the configuration of Fusion Drive. Both the drive connected via interface PCIe 2x.

    SSD SanDisk, photo iFixit

    SSD SanDisk is based on the controller Marvell 88SS9183, with whom we had previously met at the example Plextor M6e. The chip has a native PCIe interface with two lines of version 2.0, the data is transmitted via AHCI (unlike NVMe, specially designed for solid state drives). To connect to the Flash-memory chips provides eight channels. Of course, the team is supported TRIM.

    SSD itself is made in a proprietary form factor, but fans of savings can try their luck with straps party, gathering own Fusion Drive or SSD by using separately. Only with the iMac is not so easy to do, as opposed to the Mac mini.

    Marvell controller is combined with Flash-memory chip production SanDisk. Judging by the marking 05131 016G, this type of memory eX2 ABL MLC NAND, produced on 19 nm process technology. Its distinguishing feature: the part of cells operates in pseudo-SLC and serves as a cache, which speeds up the write operation and also extends the life of the chip by defragmenting requests.

    However, these cells are unlikely to many, since the drive is reserved in a standard volume of about 7% (the difference between the number of 128 GB in binary and decimal notation), which is also used for garbage collection, the substitution of defective cells and other official functions.

    On both sides of the board are placed eight sections, each of which comprises two NAND-device with a capacity of 64 Gbps (8 GB). Thus not only in storage means filled with all eight channels of the controller, but also involved alternation NAND-devices.

    In general, a very decent components for the SSD. But the verdict will leave before the end of the tests, as in the case with the Marvell controller, much depends on the firmware. Previously, we often met them at Plextor drives with firmware masterfully done. Let’s see how to show SanDisk.

    The hard drive in iMac 21.5″ is a banal 2.5-inch drive HGST Travelstar 5K1000 series with a spindle speed of 5400 rev / min. There is nothing special.

    HGST Travelstar 5K1000 1TB (HTS541010A9E662), photo iFixit

    CoreStorage – base of Fusion Drive

    Implementation of Fusion Drive is made possible by the fact that, starting with version 10.7 (Lion), in OS X is built CoreStorage – Volume Manager (volume manager), which is a software layer between the file system and storage.

    Thanks to him, and made possible the redistribution of data blocks between two physically separate devices, components array is absolutely transparent to the overlying stack software. In the terminology CoreStorage physical devices called Physical Volume and can be connected to the Logical Volume Group, has a continuous address space.

    It remains only to deploy on top of this Logical Volume, and which is presented as a normal operating system volume. Then, in the case includes at controlling the migration of data between tiers.

    Careful readers may note that in the present scheme of another entity called Logical Volume Family, which is a container for Logical Volume. Prisoners inside LVF logical volumes inherit its properties, only one of which there may be a polnodiskovoe encryption – at the expense of running the built-in OS X service FileVault 2.

    If desired, the structure can be dismantled teams from the “Terminal”, boot into rescue mode with the operating system or external drive, and then use the SSD and HDD as separate drives. On the “Mac” from Fusion Drive without any problems installing Windows in section Boot Camp.

    Last cut from the end of the logical volume and can include almost all of the address space of the hard drive, but does not apply to SSD. The installation program Windows, as set by the OS itself and subsequently sees Mac’s sections, which leaves the possibility of accidentally kill all the contents Fusion Drive.

    Subsequent installations of OS X from scratch Disk Utility, revealing the familiar iron, offer to collect it as it was with the complete destruction of data.

    How it works

    As in the case of the Fusion Drive, we are not dealing with cached and storage echelon, the first question – which originally received the recorded data in the logical volume. It was found that, as long as enough capacity SSD, only he used to write.

    With Dynamo, backend-component test tool Iometer, the disk file was created with a volume of more than 128 GB, and in the process recorded loading drives using iostat. First passes treatment exclusively to SSD, but as soon as the file size became larger capacity SSD net installed OS load completely switched to the hard disk.

    Immediately after the recording is over, the team showed fs_usage shaft calls CoreStorage, of which RdChunksCS RdMigrCS and run the data migration between the tiers of the array. The fourth column of the listing also shows that the data is moved in chunks of 128-512 Kbytes.

    Thus, due to the large-scale movements of side effect is defragmenting the data. If we sum the challenges, you get the volume in the region of 4 GB. Subsequent attempts records showed that each time on the SSD get the first 4 GB of the file, and then CoreStorage evicts the same amount of any other blocks on the hard disk.

    That is on the SSD is always a reserve of 4 GB, which provides a record svezhepostupivshih data at high speed.

    Average speed sequential read and write blocks of 256 KB with a queue of four teams with the SSD is 754 and 391 MB / s (binary), respectively. Very worthy – despite the fact that operations are over the file system. Read / write speed on HDD – 82-88 MB / s.

    But as to cause the migration of data in the opposite direction – from HDD to SSD? It was quite easy to do with the whole file. To immediately get into the address space of the hard disk, SSD was scored during a long recording of a large file, and at the same time with dd file was created with a 2 GB randomized contents of / dev / zero. After the list was immediately followed by migration, clear the volume to 4 GB of SSD.

    The first reading of the file passed with a speed of 87 MB / s (most of the loads listed on the HDD). But the second time the file has been read exclusively with SSD speed at 427 MB / s (purge guarantee the pre-cleaning of the cache). However, it is easy to note that the rate is far from the maximum: file found himself on the SSD in a fragmented state.

    Further experiments showed that the Fusion Drive can carry and the individual pieces of large files. From the middle of the file to 400 GB, which is due to the size for the most part located on the HDD, was read the range of blocks of 2 GB.

    Two passes looped reading on 30 causes displacement of the blocks on the SSD and a sharp increase overall productivity. But in order for all of the blocks were on SSD, and a read speed peaked, it took 34 passes! Interestingly, this method worked, and to record the data in the same range. In the latter case, the peak velocity was reached at the 15th passage.

    Red – reading with SSD (disk0), blue – mainly with HDD (disk1). Green – the migration of blocks after reading iteration

    It turns out that the algorithms on which Fusion Drive selects the data for migration, to act as a file and block level. And the files have priority. Access to selected blocks within files CoreStorage regards as atypical scenario and need long to require that these units were promoted.

    Well, iMac – not the best candidate for the role of the database server, and the preference of entire files contributes less fragmentation.

    In the next step we will test the SSD as part of Fusion Drive separately and compare it to other high-performance SSDs with PCIe interface or SATA 6 Gbit / s.

    Test Procedure

    Testing is carried out in the operating system Microsoft Windows 8.1 Professional x64 with Update, correctly recognize and serving modern SSDs. This means that during the test, as for normal everyday use SSD, TRIM command support and are actively involved.

    Performance measurement is performed with the drives that are in “used” condition, which reached their pre-filling data. Before each test drives are cleaned and serviced by a team of TRIM. Between individual tests aged 15 minute pause, designated for the correct development of the technology of garbage collection. In all tests, unless otherwise stated, are used incompressible randomized data.

    Used applications and tests:

    Iometer 1.1.0

    1. Measuring the speed of sequential read and write data in blocks of 256 KB (the most typical block size in successive operations in desktop tasks). Speed ​​estimation is performed for one minute, after which the average is calculated.
    2. Measuring the speed of random read and write block size of 4 KB (a block size used in the vast majority of real operations). The test is performed twice – no queue and queue requests depth of 4 teams (typical desktop applications, actively working with an extensive file system). Data blocks are aligned relative to the page of flash memory drives. Speed ​​estimation is performed for three minutes, after which the average is calculated.
    3. Establishing the velocity dependence of the random read and write when working drive with a 4-Kbyte blocks of queue depth (ranging from one to 32 teams). Data blocks are aligned relative to the page of flash memory drives. Speed ​​estimation is performed for three minutes, after which the average is calculated.
    4. Establishing the velocity dependence of the random read and write when working with storage units of different sizes. Blocks are used in volume from 512 bytes to 256 bytes. The depth of the queue for the test is 4 teams. Data blocks are aligned relative to the page of flash memory drives. Speed ​​estimation is performed for three minutes, after which the average is calculated.
    5. Measuring performance in mixed multi-threaded load and establish its dependence on the ratio of read and write operations. Uses serial read and write blocks of 128 KB, performed in two independent streams. The ratio between the read and write operations varies in increments of 10 percent. Speed ​​estimation is performed for three minutes, after which the average is calculated.
    6. Research falling SSD performance when processing a continuous stream of random write operations. To check how much falls on the SSD write speed as the filling, we gradually clog it with random data at the block level and conduct tests using Iometer random write 4KB blocks with a queue depth 4. Then the command is sent to the disk TRIM (with the utility diskpart created and formatted partition on the entire volume of the disc) and again measured the speed record.

    Testbed

    Devices selected for comparison with the Fusion Drive, were tested on a computer with a motherboard ASUS Z97-Pro, Core i5-4590 processor with integrated graphics core Intel HD Graphics 4600, and 4GB of RAM DDR3-1600 MHz.

    Drives with SATA-interface connected to the controller SATA 6 Gb / s, built-in motherboard chipset, and working in AHCI. Drives with PCI Express interface installed in the full-slot PCI Express 3.0 x16. Used driver Intel Rapid Storage Technology (RST).

    The volume and the data transmission rate specified in the benchmark binary units (1 KB = 1024 bytes).

    Test participants:

    • Apple SanDisk PCIe SSD 128 GB
    • OCZ Vector 150 240 GB
    • Plextor M6 Pro 256 GB
    • Plextor M6e 256 GB
    • Samsung 850 Pro 256 GB

    Performance

    Sequential read/write, Iometer

    It can be seen that for the drive with native PCIe interface card SanDisk, installed in our iMac, stars from the sky is not enough. Sequential read speed 256 KB blocks with a queue depth of one team does not exceed the figure of merit for tires SSD SATA 6 Gb / s.

    Plextor M6e, built on the same controller, shows a much better performance. However, the result is discouraging SanDisk clearly connected with a short queue in this test.

    When reading in four colossal team performance and actually rests within the capabilities of the controller used.

    But the lamentable result of the test on the recording speed was predictable. 256 GB model, taken for comparison, are interleaved four NAND-devices (three in the case of Samsung 850 Pro) in the channels of the controller, while SanDisk – only double alternation.

    Random read/write, Iometer

    Another test, in which the board is exposed SanDisk length queue. When reading in a single command on the result funny to watch, and when the four teams in line SSD is not something that is very much separated from most of the contenders, including Plextor M6e. The capacity of the host interface here is not critical, so unattainable for SanDisk leader is the Samsung 850 Pro.

    In the test for random write speed of the transition to a long queue does not help drive SanDisk. Performance again very modest.

    The speed of random read short blocks SanDisk although it can not compare with leading competitors (which is what Samsung Pro 850 and Plextor M6 Pro), but at least it keeps up a decent SSD for tires SATA 6 Gb / s – OCZ Vector 150. When requesting larger blocks of storage is nothing outstanding, and only in blocks of 256 KB achieves fully competitive performance.

    The speed of any record with a sufficiently long queue again determined in parallel configuration Flash-memory. Drive capacity of 128 GB here can not count on that. And so it happened: more capacious rivals show more than double the performance compared with the card SanDisk, especially if requested by the larger units and, accordingly, strong component of procedure of reading data, not access them.

    Degradation and recovery performance

    Note that even a small filling (96 GB free) already affects performance SSD. However, the absolute values ​​of the difference is small, and as further recording velocity changes little until the moment when there is available only 8 gigabytes. Then – the predictable collapse of productivity. TRIM works flawlessly, although complete purification of cells need to wait (15 minutes is enough).

    Conclusions

    Like it or not, and at the same time to make a quick, concise and cheapest hybrid drive is not possible (at least in today’s reality). But if you donate a third of these objectives, opens up interesting possibilities.

    Before buyers “Mac” is a choice between the Fusion Drive configuration 128GB SSD + 1TB HDD or SSD only 256 GB for the same money. And in fact, the choice depends on how much you have files that you want to access fast.

    Users who do not shake anything major or are used to keep the warehouse in external repositories, can never feel that your computer is not “clean” SSD: while you can, Fusion Drive does not touch the hard drive at all.

    In a more complex situation where many gigabytes of digital belongings are on the same volume, Apple’s approach to hybrid storage more efficient than anything we came across before.

    Firstly, we do not lose in the total array as SSD and HDD form a single address space.

    Secondly, migration between tiers very aggressive. Fusion Drive always keeps on the free 4GB SSD, providing quick entry of newcomers files. Promotion of files from the HDD to the SSD comes on again.

    Furthermore, the system takes into account both the activity of the block, and at the file level, preventing clogging SSD scattered blocks. Ultimately, even with active rotation data quickly occupy their rightful place.

    In general, finally someone made a hybrid drive that does not disappoint the user, taste the delights full SSD. The only complaint: $ 200 for the SSD of 128 GB, even on the bus PCIe, – it’s a little expensive for 2014.

    In addition, we would like to see the option to SSD 256 GB. That would be a truly versatile option that would satisfy the request at the same time a substantial amount of high-speed memory, hard drive and software that takes care of the distribution files.

    Should also be said about the choice of components for the Fusion Drive. To the hard drive with a spindle speed of 5400 rev / min no claims – if the SSD 7200 rev / min here would be superfluous. SSD from SanDisk also not lose face and are able to use the interface bandwidth PCIe.

    But it should be noted that the configuration with 128 GB of Flash-memory is itself inevitably inferior in performance more capacious SSD.

    In addition, the firmware card SanDisk has such a feature that at light load with a short burst of speed command is pretty mediocre for SSD bus PCIe. Perhaps drive Samsung, which Apple also uses the latest in “Poppies” spared from this drawback.

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