Primer: How data is stored on-disk with ZFS

As with a lot of things at this blog, I’m largely writing this to confirm and solidify my own knowledge. I tend to be pretty firm on how disks relate to vdevs, and vdevs relate to pools… but once you veer down deeper into the direct on-disk storage, I get a little hazier. So here’s an attempt to remedy that, with citations, for my benefit (and yours!) down the line.

Top level: the zpool

The zpool is the topmost unit of storage under ZFS. A zpool is a single, overarching storage system consisting of one or more vdevs. Writes are distributed among the vdevs according to how much FREE space each vdev has available – you may hear urban myths about ZFS distributing them according to the performance level of the disk, such that “faster disks end up with more writes”, but they’re just that – urban myths. (At least, they’re only myths as of this writing – 2018 April, and ZFS through 7.5.)

A zpool may be created with one or more vdevs, and may have any number of additional vdevs zpool added to it later – but, for the most part, you may not ever remove a vdev from a zpool. There is working code in development to make this possible, but it’s more of a “desperate save” than something you should use lightly – it involves building a permanent lookup table to redirect requests for records stored on the removed vdevs to their new locations on remaining vdevs; sort of a CNAME for storage blocks.

If you create a zpool with vdevs of different sizes, or you add vdevs later when the pool already has a substantial amount of data in it, you’ll end up with an imbalanced distribution of data that causes more writes to land on some vdevs than others, which will limit the performance profile of your pool.

A pool’s performance scales with the number of vdevs within the pool: in a pool of n vdevs, expect the pool to perform roughly equivalently to the slowest of those vdevs, multiplied by n. This is an important distinction – if you create a pool with three solid state disks and a single rust disk, the pool will trend towards the IOPS performance of four rust disks.

Also note that the pool’s performance scales with the number of vdevs, not the number of disks within the vdevs. If you have a single 12 disk wide RAIDZ2 vdev in your pool, expect to see roughly the IOPS profile of a single disk, not of ten!

There is absolutely no parity or redundancy at the pool level. If you lose any vdev, you’ve lost the entire pool, plain and simple. Even if you “didn’t write to anything on that vdev yet” – the pool has altered and distributed its metadata accordingly once the vdev was added; if you lose that vdev “with nothing on it” you’ve still lost the pool.

It’s important to realize that the zpool is not a RAID0; in conventional terms, it’s a JBOD – and a fairly unusual one, at that.

Second level: the vdev

A vdev consists of one or more disks. Standard vdev types are single-disk, mirror, and raidz. A raidz vdev can be raidz1, raidz2, or raidz3. There are also special vdev types – log and l2arc – which extend the ZIL and the ARC, respectively, onto those vdev types. (They aren’t really “write cache” and “read cache” in the traditional sense, which trips a lot of people up. More about that in another post, maybe.)

A single vdev, of any type, will generally have write IOPS characteristics similar to those of a single disk. Specifically, the write IOPS characteristics of its slowest member disk – which may not even be the same disk on every write.

All parity and/or redundancy in ZFS occurs within the vdev level.

Single-disk vdevs

This is as simple as it gets: a vdev that consists of a single disk, no more, no less.

The performance profile of a single-disk vdev is that of, you guessed it, that single disk.

Single-disk vdevs may be expanded in size by replacing that disk with a larger disk: if you zpool attach a 4T disk to a 2T disk, it will resilver into a 2T mirror vdev. When you then zpool detach the 2T disk, the vdev becomes a 4T vdev, expanding your total pool size.

Single-disk vdevs may also be upgraded permanently to mirror vdevs; just zpool attach one or more disks of the same or larger size.

Single-disk vdevs can detect, but not repair, corrupted data records. This makes operating with single-disk vdevs quite dangerous, by ZFS standards – the equivalent, danger-wise, of a conventional RAID0 array.

However, a pool of single-disk vdevs is not actually a RAID0, and really shouldn’t be referred to as one. For one thing, a RAID0 won’t distribute twice as many writes to a 2T disk as to a 1T disk. For another thing, you can’t start out with a three disk RAID0 array, then add a single two-disk RAID1 array (or three five-disk RAID5 arrays!) to your original array, and still call it “a RAID0”.

It may be tempting to use old terminology for conventional RAID, but doing so just makes it that much more difficult to get accustomed to thinking in terms of ZFS’ real topology, hindering both understanding and communication.

Mirror vdevs

Mirror vdevs work basically like traditional RAID1 arrays – each record destined for a mirror vdev is written redundantly to all disks within the vdev. A mirror vdev can have any number of constituent disks; common sizes are 2-disk and 3-disk, but there’s nothing stopping you from creating a 16-disk mirror vdev if that’s what floats your boat.

A mirror vdev offers usable storage capacity equivalent to that of its smallest member disk; and can survive intact as long as any single member disk survives. As long as the vdev has at least two surviving members, it can automatically repair corrupt records detected during normal use or during scrubbing – but once it’s down to the last disk, it can only detect corruption, not repair it. (If you don’t scrub regularly, this means you may already be screwed when you’re down to a single disk in the vdev – any blocks that were already corrupt are no longer repairable, as well as any blocks that become corrupt before you replace the failed disk(s).

You can expand a single disk to a mirror vdev at any time using the zpool attach command; you can also add new disks to an existing mirror in the same way. Disks may also be detached and/or replaced from mirror vdevs arbitrarily. You may also expand the size of an individual mirror vdev by replacing its disks one by one with larger disks; eg start with a mirror of 2T disks, then replace one disk with a 4T disk, wait for it to resilver, then replace the second 2T disk with another 4T disk. Once there are no disks smaller than 4T in the vdev, and it finishes resilvering, the vdev will expand to the new 4T size.

Mirror vdevs are extremely performant: like all vdevs, their write IOPS are roughly those of a single disk, but their read IOPS are roughly those of n disks, where n is the number of disks in the mirror – a mirror vdev n disks wide can read blocks from all n members in parallel.

A pool made of mirror vdevs closely resembles a conventional RAID10 array; each has write IOPS similar to n/2 disks and read IOPS similar to disks, where n is the total number of disks. As with single-disk vdevs, though, I’d advise you not to think and talk sloppily and call it “ZFS RAID10” – it really isn’t, and referring to it that way blurs the boundaries between pool and vdev, hindering both understanding and accurate communication.

RAIDZ vdevs

RAIDZ vdevs are striped parity arrays, similar to RAID5 or RAID6. RAIDZ1 has one parity block per stripe, RAIDZ2 has two parity blocks per stripe, and RAIDZ3 has three parity blocks per stripe. This means that RAIDZ1vdevs can survive loss of a single disk, RAIDZ2 can survive the loss of two disks, and RAIDZ3 vdevs can survive the loss of as many as three disks.

Note, however, that – just like mirror vdevs – once you’ve stripped away all the parity, you’re vulnerable to corruption that can’t be repaired. RAIDZ vdevs take typically take significantly longer to resilver than mirror vdevs do, as well – so you really don’t want to end up completely “uncovered” (surviving, but with no remaining parity blocks) with a RAIDZ array.

Each raidz vdev offers n-(parity*n) storage capacity, where n is the storage capacity of a single disk, and parity is the number of parity blocks per stripe. So a six-disk RAIDZ1 vdev offers the storage capacity of five disks, an eight-disk RAIDZ2 vdev offers the storage capacity of six disks, and so forth.

You may create RAIDZ vdevs using mismatched disk sizes, but the vdev’s capacity will be based around the smallest member disk. You can expand the size of an existing RAIDZ vdev by replacing all of its members individually with larger disks than were originally used, but you cannot expand a RAIDZ vdev by adding new disks to it and making it wider – a 5-disk RAIDZ1 vdev cannot be converted into a 6-disk RAIDZ1 vdev later; neither can a 6-disk RAIDZ2 be converted into a 6-disk RAIDZ1.

It’s a common misconception to think that RAIDZ vdev performance scales linearly with the number of disks used. Although throughput under ideal conditions can scale towards n-parity disks, throughput under moderate to serious load will rapidly degrade toward the profile of a single disk – or even slightly worse, since it scales down toward the profile of the slowest disk for any given operation. This is the difference between IOPS and bandwidth (and it works the same way for conventional RAID!)

RAIDZ vdev IOPS performance is generally more robust than that of a conventional RAID5 or RAID6 array of the same size, because RAIDZ offers variable stripe write sizes – if you routinely write data in records only one record wide, a RAIDZ1 vdev will write to only two of its disks (one for data, and one for parity); a RAIDZ2 vdev will write to only three of its disks (one for data, and two for parity) and so on. This can mitigate some of the otherwise-crushing IOPS penalty associated with wide striped arrays; a three-record variable stripe write to a six-disk RAIDZ vdev only lights up half the disks both when written, and later, when read – which can make the performance profile of that six-disk RAIDZ resemble that of two three-disk RAIDZ1 vdevs rather than that of a single vdev.

The performance improvement described above assumes that multiple reads and writes of the three-record stripes are being requested concurrently; otherwise the entire vdev still binds while waiting for a full-stripe read or write.

Remember that you can – and with larger servers, should – have multiple RAIDZ vdevs per pool, not just one. A pool of three eight-disk RAIDZ2 vdevs will significantly outperform a pool with a single 24-disk RAIDZ2 or RAIDZ3 vdev – and it will resilver much faster when replacing failed disks.

Third level: the metaslab

Each vdev is organized into metaslabs – typically, 200 metaslabs per vdev (although this number can change, if vdevs are expanded and/or as the ZFS codebase itself becomes further optimized over time).

When you issue writes to the pool, those writes are coalesced into a txg (transaction group), which is then distributed among individual vdevs, and finally allocated to specific metaslabs on each vdev. There’s a fairly hefty logic chain which determines exactly what metaslab a record is written to; it was explained to me (with no warranty offered) by a friend who worked with Oracle as follows:

• Is this metaslab “full”? (zfs_mg_noalloc_threshold)
• Is this metaslab excessively fragmented? (zfs_metaslab_fragmentation_threshold)
• Is this metaslab group excessively fragmented? (zfs_mg_fragmentation_threshold)
• Have we exceeded minimum free space thresholds? (metaslab_df_alloc_threshold) This one is weird; it changes the whole storage pool allocation strategy for ZFS if you cross it.
• Should we prefer lower-numbered metaslabs over higher ones? (metaslab_lba_weighting_enabled) This is totally irrelevant to all-SSD pools, and should be disabled there, because it’s pretty stupid without rust disks underneath.
• Should we prefer lower-numbered metaslab groups over higher ones? (metaslab_bias_enabled) Same as above.

You can dive into the hairy details of your pool’s metaslabs using the zdb command – this is a level which I have thankfully not personally needed so far, and I devoutly hope I will continue not to need it in the future.

Fourth level: the record

Each ZFS write is broken into records, the size of which is determined by the zfs set recordsize=command. The default recordsize is currently 128K; it may range from 512B to 1M.

Recordsize is a property which can be tuned individually per dataset, and for higher performance applications, should be tuned per dataset. If you expect to largely be moving large chunks of contiguous data – for example, reading and writing 5MB JPEG files – you’ll benefit from a larger recordsize than default. Setting recordsize=1M here will allow your writes to be less fragmented, resulting in higher performance both when making the writes, and later when reading them.

Conversely, if you expect a lot of small-block random I/O – like reading and writing database binaries, or VM (virtual machine) images – you should set recordsize smaller than the default 128K. MySQL, as an example, typically works with data in 16K chunks; if you set recordsize=16K you will tremendously improve IOPS when working with that data.

ZFS CSUMs – cryptographic hashes which verify its data’s integrity – are written on a per-record basis; data written with recordsize=1M will have a single CSUM per 1MB; data written with recordsize=8K will have 128 times as many CSUMs for the same 1MB of data.

Setting recordsize to a value smaller than your hardware’s individual sector size is a tremendously bad idea, and will lead to massive read/write amplification penalties.

Fifth (and final) level: ashift

Ashift is the property which tells ZFS what the underlying hardware’s actual sector size is. The individual blocksize within each record will be determined by ashift; unlike recordsize, however, ashift is set as a number of bits rather than an actual number.  For example, ashift=13 specifies 8K sectors, ashift=12 specifies 4K sectors, and ashift=9 specifies 512B sectors.

Ashift is per vdev, not per pool – and it’s immutable once set, so be careful not to screw it up!  In theory, ZFS will automatically set ashift to the proper value for your hardware; in practice, storage manufacturers very, very frequently lie about the underlying hardware sector size in order to keep older operating systems from getting confused, so you should do your homework and set it manually. Remember, once you add a vdev to your pool, you can’t get rid of it; so if you accidentally add a vdev with improper ashift value to your pool, you’ve permanently screwed up the entire pool!

Setting ashift too high is, for the most part, harmless – you’ll increase the amount of slack space on your storage, but unless you have a very specialized workload this is unlikely to have any significant impact. Setting ashift too low, on the other hand, is a horrorshow. If you end up with an ashift=9 vdev on a device with 8K sectors (thus, properly ashift=13), you’ll suffer from massive write amplification penalties as ZFS needs to write, read, rewrite again over and over on the same actual hardware sector. I have personally seen improperly set ashift cause a pool of Samsung 840 Pro SSDs perform slower than a pool of WD Black rust disks!

Even if you’ve done your homework and are absolutely certain that your disks use 512B hardware sectors, I strongly advise considering setting ashift=12 or even ashift=13 – because, remember, it’s immutable per vdev, and vdevs cannot be removed from pools. If you ever need to replace a 512B sector disk in a vdev with a 4K or 8K sector disk, you’ll be screwed if that vdev is ashift=9.

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Jim Salter

Mercenary sysadmin, open source advocate, and frotzer of the jim-jam.

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