- 1 Introduction
- 2 ECC Memory
- 3 Drive Interfaces
- 4 Controllers
- 5 Hard drives
- 6 NAND Flash SSDs
Storage before ZFS involved rather expensive hardware that was unable to protect against silent corruption and did not scale very well. The introduction of ZFS has enabled people to use far less expensive hardware than previously used in the industry with superior scaling. This page attempts to provide some basic guidance to people buying hardware for use in ZFS-based servers and workstations.
Hardware that adheres to this guidance will enable ZFS to reach its full potential for performance and reliability. Hardware that does not adhere to it will serve as a handicap. Unless otherwise stated, such handicaps apply to all storage stacks and are by no means specific to ZFS. Systems built using competing storage stacks will also benefit from these suggestions.
Bit flips can have fairly dramatic consequences for all computer filesystems and ZFS is no exception. No technique used in ZFS (or any other filesystem) is capable of protecting against bit flips. Consequently, ECC Memory is highly recommended.
Ordinary background radiation will randomly flip bits in computer memory, which causes undefined behavior. These are known as "bit flips". Each bit flip can have any of three possible consequences depending on which bit is flipped:
- Bit flips can have no effect.
- Bit flips that have no effect occur in unused memory.
- Bit flips can cause runtime failures.
- This is the case when a bit flip occurs in something read from disk.
- Failures are typically observed when program code is altered.
- If the bit flip is in a routine within the system's kernel or /sbin/init, the system will likely crash. Otherwise, reloading the affected data can clear it. This is typically achieved by a reboot.
- It can cause data corruption.
- This is the case when the bit is in use by data being written to disk.
- If the bit flip occurs before ZFS' checksum calculation, ZFS will not realize that the data is corrupt.
- If the bit flip occurs after ZFS' checksum calculation, but before write-out, ZFS will detect it, but it might not be able to correct it.
- It can cause metadata corruption.
- This is the case when a bit flips in an on-disk structure being written to disk.
- If the bit flip occurs before ZFS' checksum calculation, ZFS will not realize that the metadata is corrupt.
- If the bit flip occurs after ZFS' checksum calculation, but before write-out, ZFS will detect it, but it might not be able to correct it.
- Recovery from such an event will depend on what was corrupted. In the worst, case, a pool could be rendered unimportable.
- All filesystems have poor reliability in their absolute worst case bit-flip failure scenarios. Such scenarios should be considered extraordinarily rare.
SAS versus SATA
ZFS depends on the block device layer for storage. Consequently, ZFS is affected by the same things that affect other filesystems, such as driver support and non-working hardware. Consequently, there are a few things to note:
- Never place SATA disks into a SAS expander without a SAS interposer.
- If you do this and it does work, it is the exception, rather than the rule.
- Do not expect SAS controllers to be compatible with SATA port multipliers.
- This configuration is typically not tested.
- The disks could be unrecognized.
- Support for SATA port multipliers is inconsistent across Open ZFS platforms
- Linux drivers generally support them.
- Illumos drivers generally do not support them.
- FreeBSD drivers are somewhere between Linux and Illumos in terms of support.
USB Hard Drives and/or Adapters
These have problems involving sector size reporting, SMART passthrough, the ability to set ERC and other areas. ZFS will perform as well on such devices as they are capable of allowing, but try to avoid them. They should not be expected to have the same up-time as SAS and SATA drives and should be considered unreliable.
The ideal storage controller for ZFS has the following attributes:
- Driver support on major Open ZFS platforms
- Stability is important.
- High per-port bandwidth
- PCI Express interface bandwidth divided by the number of ports
- Low cost
- Support for RAID, Battery Backup Units and hardware write caches is unnecessary.
Marc Bevand's blog post From 32 to 2 ports: Ideal SATA/SAS Controllers for ZFS & Linux MD RAID contains an excellent list of storage controllers that meet these criteria. He regularly updates it as newer controllers become available.
Hardware RAID controllers
Hardware RAID controllers should not be used with ZFS. While ZFS will likely be more reliable than other filesystems on Hardware RAID, it will not be as reliable as it would be on its own.
- Hardware RAID will limit opportunities for ZFS to perform self healing on checksum failures is limited. When ZFS does RAID-Z or mirroring, a checksum failure on one disk can be corrected by treating the disk containing the sector as bad for the purpose of reconstructing the original information. This cannot be done when a RAID controller handles the redundancy unless a duplicate copy is stored by ZFS in the case that the corruption involving as metadata, the copies flag is set or the RAID array is part of a mirror/raid-z vdev within ZFS.
- Sector size information is not necessarily passed correctly by hardware RAID on RAID 1 and cannot be passed correctly on RAID 5/6. Hardware RAID 1 is more likely to experience read-modify-write overhead from partial sector writes and Hardware RAID 5/6 will almost certainty suffer from partial stripe writes (i.e. the RAID write hole). Using ZFS with the disks directly will allow it to obtain the sector size information reported by the disks to avoid read-modify-write on sectors while ZFS avoids partial stripe writes on RAID-Z by desing from using copy-on-write.
- There can be sector alignment problems on ZFS when a drive misreports its sector size. Such drives are typically NAND-flash based solid state drives and older SATA drives from the advanced format (4K sector size) transition before Windows XP EoL occurred. This can be manually corrected at vdev creation.
- It is possible for the RAID header to cause misalignment of sector writes on RAID 1 by starting the array within a sector on an actual drive, such that manual correction of sector alignment at vdev creation does not solve the problem.
- If multiple RAID arrays are formed by the same controller and one fails, the identifiers provided by the arrays exposed to the OS might become inconsistent. Giving the drives directly to the OS allows this to be avoided via naming that maps to a unique port or unique drive identifier.
- e.g. If you have arrays A, B, C and D; array B dies, the interaction between the hardware RAID controller and the OS might rename arrays C and D to look like arrays B and C respectively. This can fault pools verbatim imported from the cachefile.
- Not all RAID controllers behave this way. However, this issue has been observed on both Linux and FreeBSD.
- Controller failures can require that the controller be replaced with the same model, or in less extreme cases, a model from the same manufacturer. Using ZFS by itself allows any controller to be used.
- IO response times will be reduced whenever the OS blocks on IO operations because the system CPU blocks on a much weaker embedded CPU used in the RAID controller. This lowers IOPS relative to what ZFS could have achieved.
- If a hardware RAID controller's write cache is used, an additional failure point is introduced that can only be partially mitigated by additional complexity from adding flash to save data in power loss events. The data can still be lost if the battery fails when it is required to survive a power loss event or there is no flash and power is not restored in a timely manner. The loss of the data in the write cache can severely damage anything stored on a RAID array when many outstanding writes are cached. In addition, all writes are stored in the cache rather than just synchronous writes that require a write cache, which is inefficient, and the write cache is relatively small. ZFS allows synchronous writes to be written directly to flash, which should provide similar acceleration to hardware RAID and the ability to accelerate many more in-flight operations.
- Behavior during RAID reconstruction when silent corruption damages data is undefined. There are reports of RAID 5 and 6 arrays being lost during reconstruction when the controller encounters silent corruption. ZFS' checksums allow it to avoid this situation by determining if not enough information exists to reconstruct data. In which case, the file is listed as damaged in zpool status and the system administrator has the opportunity to restore it from a backup.
- The controller's firmware is an additional layer of complexity that cannot be inspected by arbitrary third parties. The ZFS source code is open source and can be inspected by anyone.
One might be inclined to try using single-drive RAID 0 arrays to try to use a RAID controller like a HBA, but this is not recommended for many of the reasons listed for other hardware RAID types. It is best to use a HBA instead of a RAID controller, for both performance and reliability.
Historically, all hard drives had 512-byte sectors, with the exception of some SCSI drives that could be modified to support slightly larger sectors. In 2009, the industry migrated from 512-byte sectors to 4096-byte "Advanced Format" sectors. Since Windows XP is not compatible with 4096-byte sectors or drives larger than 2TB, some of the first advanced format drives implemented hacks to maintain Windows XP compatibility.
- The first advanced format drives on the market misreported their sector size as 512-bytes for Windows XP compatibility. As of 2013, it is believed that such hard drives are no longer in production. Advanced format hard drives made during or after this time should report their true physical sector size.
- Drives storing 2TB and smaller might have a jumper that can be set to map all sectors off by 1. This to provide proper alignment for Windows XP, which started its first partition at sector 63. This jumper setting should be off when using such drives with ZFS.
As of 2014, there are still 512-byte and 4096-byte drives on the market, but they are known to properly identify themselves unless behind a USB to SATA controller. Replacing a 512-byte sector drive with a 4096-byte sector drives in a vdev created with 512-byte sector drives will adversely affect performance. Replacing a 4096-byte sector drive with a 512-byte sector drive will have no negative effect on performance.
Error recovery control
ZFS is said to be able to use cheap drives. This was true when it was introduced and hard drives supported Error recovery control. Since ZFS' introduction, error recovery control has been removed from low-end drives from certain manufacturers, most notably Western Digital. Consistent performance requires hard drives that support error recovery control.
Hard drives store data using small polarized regions a magnetic surface. Reading from and/or writing to this surface poses a few reliability problems. One is that imperfections in the surface can corrupt bits. Another is that vibrations can cause drive heads to miss their targets. Consequently, hard drive sectors are composed of three regions:
- A sector number
- The actual data
The sector number and ECC enables hard drives to detect and respond to such events. When either event occurs during a read, hard drives will retry the read many times until they either succeed or conclude that the data cannot be read. The latter case can take a substantial amount of time and consequently, IO to the drive will stall.
Enterprise hard drives and some consumer hard drives implement a feature called Time-Limited Error Recovery (TLER) by Western Digital, Error Recovery Control (ERC) by Seagate and Command Completion Time Limit by Hitachi and Samsung, which permits the time drives are willing to spend on such events to be limited by the system administrator.
Drives that lack such functionality can be expected to have arbitrarily high limits. Several minutes is not impossible. Drives with this functionality typically default to 7 seconds. ZFS does not currently adjust this setting on drives. However, it is advisable to write a script to set the error recovery time to a low value, such as 0.1 seconds until ZFS is modified to control it. This must be done on every boot.
High RPM drives have lower seek times, which is historically regarded as being desirable. They increase cost and sacrifice storage density in order to achieve what is typically no more than a factor of 6 improvement over their lower RPM counterparts.
To provide some numbers, a 15k RPM drive from a major manufacturer is rated for 3.4 millisecond average read and 3.9 millisecond average write. Presumably, this number assumes that the target sector is at most half the number of drive tracks away from the head and half the disk away. Being even further away is worst-case 2 times slower. Manufacturer numbers for 7200 RPM drives are not available, but they average 13 to 16 milliseconds in empirical measurements. 5400 RPM drives can be expected to be slower.
ARC and ZIL are able to mitigate much of the benefit of lower seek times. Far larger increases in IOPS performance can be obtained by adding additional RAM for ARC, L2ARC devices and SLOG devices. Even higher increases in performance can be obtained by replacing hard drives with solid state storage entirely. Such things are typically more cost effective than high RPM drives when considering IOPS.
Drives with command queues are able to reorder IO operations to increase IOPS. This is called Native Command Queuing on SATA and Tagged Command Queuing on PATA/SCSI/SAS. ZFS stores objects in metaslabs and it can use several metastabs at any given time. Consequently, ZFS is not only designed to take advantage of command queuing, but good ZFS performance requires command queuing. Almost all drives manufactured within the past 10 years can be expected to support command queuing. The exceptions are:
- Consumer PATA/IDE drives
- First generation SATA drives, which used IDE to SATA translation chips, from 2003 to 2004.
- SATA drives operating under IDE emulation that was configured in the system BIOS.
Each Open ZFS system has different methods for checking whether command queuing is supported. On Linux, `hdparm -I /path/to/device | grep Queue` is used. On FreeBSD, `camcontrol identify $DEVICE` is used.
NAND Flash SSDs
As of 2014, Solid state storage is dominated by NAND-flash and most articles on solid state storage focus on it exclusively. As of 2014, the most popular form of flash storage used with ZFS involve drives with SATA interfaces. Enterprise models with SAS interfaces are beginning to become available.
Power Failure Protection
Flash drives used for top-level vdevs or SLOG devices should have power failure protection. A non-exhaustive list of drives with power failure protection is as follows:
- Crucial M500
- Crucial M550
- Intel 320/330/335
- Intel 710
- Intel 730
- Intel DC S3500/S3510/S3610/S3700/S3710
- Samsung SM1625
- Samsung SM843T (do not confuse with SM843)
- Samsung 845DC Evo
- Samsung 845DC Pro
- High sustained write IOPS
- Samsung PM853T
On-flash data structures are highly complex and consequently, vulnerable to corruption. Such corruption can result in the loss of *all* drive data and an event such as a PSU failure can result in multiple drives simultaneously failing. Since the drive firmware is not available for review, the only reasonable conclusion is that all drives that lack hardware features to avoid power failure events cannot be trusted. Therefore, such drives are only suitable for use as L2ARC.
The smallest unit on a NAND chip that can be written is a flash page. The first NAND-flash SSDs on the market had 4096-byte pages. Further complicating matters is that the the page size has been doubled twice since then. NAND flash SSDs *should* report these pages as being sectors, but so far, all of them incorrectly report 512-byte sectors for Windows XP compatibility. The consequence is that we have a similar situation to what we had with early advanced format hard drives.
As of 2014, most NAND-flash SSDs on the market have 8192-byte page sizes. However, models using 128-Gbit NAND from certain manufacturers have a 16384-byte page size. Maximum performance requires that vdevs be created with correct ashift values (13 for 8192-byte and 14 for 16384-byte). However, not all Open ZFS platforms support this. The Linux port supports ashift=13, while others are limited to ashift=12 (4096-byte).
ATA TRIM / SCSI UNMAP
At this time, only the FreeBSD port has support for sending block discard commands to vdevs to generate appropriate ATA TRIM and/or SCSI UNMAP commands. It should be noted that this is a separate case from discard on zvols or hole punching on filesystems. Those work regardless of whether ATA TRIM / SCSI UNMAP is sent to the actual block devices.
ATA TRIM Performance Issues
The ATA TRIM command in SATA 3.0 and earlier is a non-queued command. Issuing a TRIM command on a SATA drive conforming to SATA 3.0 or earlier will cause the drive to drain its IO queue and stop servicing requests until it finishes, which hurts performance. SATA 3.1 removed this limitation, but very few SATA drives on the market are conformant to SATA 3.1 and it is difficult to distinguish them from SATA 3.0 drives. At the same time, SCSI UNMAP has no such problems.