OK, added some additional data for folk who want to follow in my footsteps re: implementing a sVDEV now. If someone has a better approach for planning, (i.e. determining the minimum sizes needed for either sVDEV partition) please let me know. I hope this is a good start towards a resource in the future.
Why consider a sVDEV?
A sVDEV has the potential to significantly speed up the operations of a HDD pool by putting small files and pool metadata into a faster special VDEV (sVDEV), which usually consists of SSDs. You can also designate specific datasets to reside on a sVDEV by setting record size in the dataset configuration page to equal the sVDEV small file cutoff. Thus, the common practice of segregating pools by hard drive type (SSD vs. HDD) is no longer necessary.
What about L2ARC?
The main difference between Second Level Adaptive Replacement Cache (L2ARC) and sVDEV is that while L2ARC can cache frequently-accessed files, or metadata, or files and metadata, it is read-only, redundant, and it also needs to get “hot”. Unlike sVDEV, the L2ARC has to “miss” having a file or metadata in cache before the miss is noted and might be read into the L2ARC for later reuse. L2ARC also requires RAM to function, so the common recommendation here is not to enable L2ARC unless you have a minimum 64GB of RAM available to TrueNAS.
Based on my limited testing, it took my NAS about three passes with rsync for a metadata=only L2ARC to cache all the metadata it needed.. As you can see from my test results, using L2ARC solely for metadata sped up the rsync task significantly. For an in-depth discussion of L2ARC, see this TrueNAS documentation hub page. Happily, L2ARC can be made persistent, allowing a “hot” cache to survive reboots. Because L2ARC is redundant, any SSD will do, as long as it’s read performance is decent.
Unlike L2ARC, the sVDEV doesn’t need more RAM and it’s 100% hot from the start since all metadata will reside on it. Additionally, the sVDEV will also host small files, which are the worst-performing category for HDDs to deal with. Unlike the L2ARC, your pool will die if the sVDEV stops functioning. For a L2ARC-sVDEV comparison, see my results from a few years ago comparing the impact of L2ARC and sVDEVs in my rsync use case.
About record sizes and small file cutoffs…
What constitutes a small file depends in part on the recordsize used in your pool / dataset. By default, TrueNAS uses 128k recordsizes but you can adjust them on a per-dataset basis, if you wish (See Storage/Pool/Dataset triple-dot on the right of the GUI). Larger record sizes (up to 1M) used in conjunction with compression are great for media files. Smaller record sizes are helpful for databases and like use cases.
For simple large-file data transfers, adjusting the record size up to 1M with zstd compression and adding the sVDEV, made transfer speeds to my pool jump from about 250MB/s to 400MB/s. Not bad for a single-VDEV Z3 pool.
Important: the pool depends on the sVDEV to function, so if the sVDEV dies, your pool dies also. Take the same care with sVDEVs re: redundancy and resilience as you did with the other VDEVs! Do not use crummy SSDs for the sVDEV unless you don’t care about pool life expectancy..
Planning
Ideally, a special VDEV (sVDEV) is planned carefully in advance and is added to the pool before the pool is populated with data. In order to get moved to the sVDEV, a small file has to be copied into the pool or rebalanced in-situ, so figuring out how much storage the sVDEV of a new pool needs in advance is better than adding one later and rebalancing everything just to migrate small files into the sVDEV.
Similarly, you will want to adjust the record size of each data set ideally in advance to figure out what the best balance between record size and sVDEV use is. The good news is that sVDEV cutoffs can be adjusted by dataset, so there is plenty of flexibility.
This feature is also interesting in that you can make datasets that use the same small file cutoff as the recordsize, ensuring that the entire dataset sits on flash drives only. Thus, a sVDEV can selectively host all data that needs speed (databases, metadata, etc.) all as part of a common pool vs. setting up two pools (one flash, the other HDD).
Required sVDEV Size
The needed sVDEV size has be determined by how much metadata and small files data needs to be held. By default, the sVDEV features a 25/75% partition split between metadata and small files, respectively. It can be adjusted by adjusting the zfs_special_class_metadata_reserve_pct parameter (thank you, @HoneyBadger!). The sVDEV does not like to be filled more than 75% by default and you also have to allow for pool expansion in the future. Some extrapolation is likely needed.
Determining Metadata Space Needs
There is a rule of thumb that metadata consumes about 0.3% of the pool space but this will vary by use case. NAS’ hosting only large video files will have comparatively little metadata compared to someone who makes a non-archived copy of a MacOS drive. If you have the time, there are CLI commands to allow the user to determine the size of the metadata exactly. In TrueNAS Core as of 13.0U6, the command to determine your metadata space needs is
zdb -LbbbA -U /data/zfs/zpool.cache poolname
That will spit out a series of tables. One will be long… so long in fact that I’ve truncated mine to only show the top and the bottom while cutting out the middle.
Blocks LSIZE PSIZE ASIZE avg comp %Total Type
- - - - - - - unallocated
2 32K 8K 24K 12K 4.00 0.00 object directory
.... SNIP! .....
89.0K 2.80G 359M 718M 8.06K 8.00 0.00 L2 Total
1.61M 51.6G 11.6G 23.2G 14.4K 4.44 0.08 L1 Total
140M 17.3T 17.2T 30.1T 220K 1.01 99.92 L0 Total
142M 17.4T 17.2T 30.1T 218K 1.01 100.00 Total
Draw your attention to the line with L1 total, i.e. three lines up from the bottom. There is our metadata information i.e. 23.2GB or 0.08% of my pool was metadata. Extrapolating across 50TB of pool capacity in my use case, that comes out to about 40GB of metadata needs.
Once TrueNAS transitions to OpenZFS 2.2.0 or higher, the current metadata space needs will be even easier to determine via zdb -bbbs with a nice summary at the bottom, see here. Now you know one minimum.
Small File Space Needs
The zdb -LbbbA -U /data/zfs/zpool.cache poolname command will also spit out the answer for your small file needs by publishing something called the “Block Size Histogram”. It summarizes the block sizes used, i.e. how many of each type can be found in your pool today.
Block Size Histogram
block psize lsize asize
size Count Size Cum. Count Size Cum. Count Size Cum.
512: 422K 211M 211M 422K 211M 211M 0 0 0
1K: 113K 123M 334M 113K 123M 334M 0 0 0
2K: 65.7K 175M 510M 65.7K 175M 510M 0 0 0
4K: 1.92M 7.71G 8.21G 119K 554M 1.04G 1.01M 4.04G 4.04G
8K: 383K 3.56G 11.8G 39.6K 436M 1.46G 1.77M 14.6G 18.6G
16K: 765K 13.2G 25.0G 316K 5.18G 6.64G 297K 5.87G 24.5G
32K: 368K 16.5G 41.5G 1.80M 59.3G 65.9G 585K 18.8G 43.4G
64K: 622K 56.3G 97.8G 34.0K 2.73G 68.6G 386K 32.4G 75.8G
128K: 137M 17.1T 17.2T 139M 17.3T 17.4T 138M 30.0T 30.1T
256K: 0 0 17.2T 0 0 17.4T 605 164M 30.1T
512K: 0 0 17.2T 0 0 17.4T 0 0 30.1T
1M: 0 0 17.2T 0 0 17.4T 0 0 30.1T
2M: 0 0 17.2T 0 0 17.4T 0 0 30.1T
4M: 0 0 17.2T 0 0 17.4T 0 0 30.1T
8M: 0 0 17.2T 0 0 17.4T 0 0 30.1T
16M: 0 0 17.2T 0 0 17.4T 0 0 30.1T
Pay attention to the column “Cum.” to the right of the table. That shows how much room the small files are taking up in your pool on a cumulative basis. None of my files show a record size about 128k, since that is block size that this pool was created with. It’s also why the sVDEV cutoff has to be set below the pool record size.
Once you have your small file distribution, you need to determine where to set your cutoff re: what small files to send to the sVDEV vs. the general pool. By default, I chose 32kB which consumed 18.8GB of metadata needs. Plenty of space for the pool to grow. Extrapolate your data needs and see what kind of storage you might need.
For example, if my pool is 20% full and small files needs make up for 18GB as shown above, then a full pool would need 5x as much sVDEV space for small files, i.e. 90GB. If I leave the small files / metadata ratio at the default 75/25% ratio, then my sVDEV pool has to be at least 120GB. However, you might find that by adjusting the record sizes that the diversity of your pool will increase and more data could be stored in the sVDEV. If you suspect that you will host more smaller files in the future, you would do well to select a much larger sVDEV to account for expansion in the future.
sVDEV drive selection
At minimum, choose SSDs that can handle the workload and a proven track record re: endurance. Power Loss Protection (PLP) is also a nice to have, though that can get expensive. I went for Intel S3610 SATA SSDs that (though used) have incredible write-endurance. I also bought two spares. I qualified all my SSDs before using them, set aside the spares in caddies, ready to mount if one of the four SSDs I use for a sVDEV develop a failure and needs to be replaced.
Because a loss of the sVDEV will result in a pool loss, you will want to mount sVDEV SSDs in mirrors, at minimum in a 2-way. I use a four-way mirror for my Z3 pool.
Implementation
Yes, the sVDEV can be enabled / attached / assigned at the GUI level, see here. Make sure all the SSDs you need are in the sVDEV, set up as mirrors, etc.
The small file record size cutoff can be set via the GUI - select “Storage” then “Pools”, followed by going over the pool/dataset table. The pool or individual dataset small file cutoff can be adjusted by opening “edit option” (see three dots to the right of each pool or dataset name in the menu) and adjusting the “Metadata small block size”. If you do it for the pool, the datasets will inherit the pool value by default. You can also do this via the CLI, per this Level 1 post, the command is
zfs set special_small_blocks=64K poolname
or if you want to vary small file cutoffs on a dataset basis:
zfs set special_small_blocks=128K poolname/dataset
If you are about to enjoy a new pool of content, congratulations, you are pretty much done. If you have an extant pool, the small-files benefit won’t really start to accrue until you move the small files onto the sVDEV.
This is also a great time to brush up on recordsize and to determine if you should adjust the record sizes of particular pools to better suit the data in them - you can do this in the GUI by selecting, Storage → Pools and then individual datasets. Lastly, there may be some small pools (think iocage jails) where likely all data should reside on the sVDEV. Go ahead and tune the recordsize cutoff by dataset to your liking.
There is no GUI rebalancing command, though we may get one if ZFS VDEV expansion ever becomes a reality in TrueNAS. In the meantime, there is an excellent script written in bash that can do the rebalancing for you. Before you rebalance, back up, turn off snapshot and then replication tasks. I also had to delete all snapshots since it becomes very difficult to see the impact of record size changes and rebalancing efforts if snapshots pollute the data.
In TrueNAS Core, you have to “su” to become the owner of the data before you run the script, I’d also advise to tmux the session to ensure the command can actually execute across the whole dataset. After installing the script in my root folder, my steps are as follows:
- cd /mnt/pool/dataset
- ls -al (to see who the owner is)
- tmux
- su owner
- bash /root/zfs-inplace-rebalancing.sh --checksum true --passes 1 /pool/path/to/rebalance
then CTRL-B and then “d” to detach.
It will run for a while, and as it does, you’ll watch the pool start to redistribute data, especially if you have also tuned the recordsize to be larger for some pools like image, archive, or video content. Hopefully, you’ll see the big sets of data migrate to larger recordsizs, allowing you to fine tune the cutoff limits for the sVDEV recordsizes, storing as much small file data on the sVDEVs as possible to maximize the speed boost.
You may have to run the rebalancing script a few times. Only when your pool is “perfect” turn both Snapshot and Replication tasks back on. I would also hit the “Add” button in Storage / Snapshot GUI at this point to manually execute a snapshot. Replication tasks can only start if there is a extant snapshot. Then go to Tasks / Replication and “Run Now” your replication tasks to verify they’re still OK.
Maintenance
Sadly, the dashboard currently gives ZERO clues re: the sVDEV other than whether the disks are still available and/or whether they passed their latest SMART test. For example: zero data on how full the sVDEV partitions are. If you go into reporting, you can get a glimpse into I/O of the sVDEV drives but not on a group basis, which would be helpful if your sVDEV pool contains multiple mirrors.
Even a basic check to see how full the sVDEV is requires CLI commands (i.e. use zpool list -vvv poolname to see how full each VDEV is [thank you, @NickF1227!).
I find the lack of good documentation somewhat disheartening, sVDEVs offer a significant performance gain but have to be handled with care. Documenting the issues, implementing GUI commands, etc. should have happened by now.
Some other considerations
I would ensure that the remote NAS for a replication task has the exact same settings re: recordsize, compression, sVDEV small file cutoff (if fitted), etc. on a dataset by dataset basis as the primary NAS. If your remote NAS already has content, I’d either wipe and start over (safest option) or nuke all remote snapshots, change the recordsizes, etc. to match the primary NAS, rebalance, and only then re-enable replication. (seems like a less safe option)
So what though?
Well, I have found the implementation of sVDEV on my pool to enable a improvement over L2ARC in terms of metadata, even if a L2ARC is “hot”, persistent, and metadata-only (the L2ARC only really helps for read operations re: metadata, not writes, which have to go to the slow HDD pool). Between sVDEV, recordsize=1M, compression=zstd, and a rebalance, my sustained large-file pool write speed has risen to 400MB/s, well above the expected 250MB/s limit I used to experience. The
zdb -LbbbA -U /data/zfs/zpool.cache poolname
command now executes in less than two minutes and the amount of files stored in the small files sVDEV has shot up considerably. Meanwhile, my NAS’ metadata needs dropped to just 0.03% of the pool.
17.0T completed (331016MB/s) estimated time remaining: 0hr 00min 00sec
bp count: 13671449 ganged count: 0
bp logical: 12053360033792 avg: 881644
bp physical: 11620838932480 avg: 850007 compression: 1.04
bp allocated: 18659308601344 avg: 1364837 compression: 0.65
bp deduped: 0 ref>1: 0 deduplication: 1.00
Normal class: 18553586663424 used: 23.21%
Special class 106690252800 used: 6.68%
Embedded log class 0 used: 0.00%
additional, non-pointer bps of type 0: 131
number of (compressed) bytes: number of bps
and
Blocks LSIZE PSIZE ASIZE avg comp %Total Type
- - - - - - - unallocated
2 32K 8K 24K 12K 4.00 0.00 object directory
1 32K 8K 24K 24K 4.00 0.00 L1 object array
77 38.5K 38.5K 924K 12K 1.00 0.00 L0 object array
78 70.5K 46.5K 948K 12.2K 1.52 0.00 object array
------ SNIP! ------
586K 18.3G 2.61G 5.23G 9.15K 7.00 0.03 L1 Total
12.5M 10.9T 10.6T 17.0T 1.36M 1.04 99.97 L0 Total
13.0M 11.0T 10.6T 17.0T 1.30M 1.04 100.00 Total
and
Block Size Histogram
block psize lsize asize
size Count Size Cum. Count Size Cum. Count Size Cum.
512: 238K 119M 119M 238K 119M 119M 0 0 0
1K: 80.2K 99.0M 218M 80.2K 99.0M 218M 0 0 0
2K: 126K 334M 552M 126K 334M 552M 0 0 0
4K: 849K 3.36G 3.90G 97.3K 526M 1.05G 458K 1.79G 1.79G
8K: 96.8K 914M 4.80G 60.3K 662M 1.70G 916K 7.29G 9.08G
16K: 135K 2.42G 7.21G 229K 3.86G 5.56G 63.5K 1.28G 10.4G
32K: 197K 9.10G 16.3G 701K 23.6G 29.1G 267K 11.1G 21.5G
64K: 144K 11.8G 28.2G 31.6K 2.49G 31.6G 135K 11.0G 32.5G
128K: 561K 75.9G 104G 620K 79.5G 111G 566K 119G 152G
256K: 185K 67.5G 172G 57.1K 20.3G 131G 173K 63.0G 215G
512K: 303K 214G 386G 58.3K 40.9G 172G 105K 85.4G 300G
1M: 10.2M 10.2T 10.6T 10.8M 10.8T 11.0T 10.4M 16.7T 17.0T
2M: 0 0 10.6T 0 0 11.0T 0 0 17.0T
4M: 0 0 10.6T 0 0 11.0T 0 0 17.0T
8M: 0 0 10.6T 0 0 11.0T 0 0 17.0T
16M: 0 0 10.6T 0 0 11.0T 0 0 17.0
NAME SIZE ALLOC FREE CKPOINT EXPZ FRAG CAP DEDUP HEALTH ALTROOT
pool 74.2T 17.0T 57.2T - - 0% 22% 1.00x ONLINE /mnt
raidz3-0 72.7T 16.9T 55.9T - - 0% 23.2% - ONLINE
gptid/e6a453ca-90cf-11eb-acc9-ac1f6b738b00 9.09T - - - - - - - ONLINE
gptid/e7dfdfc1-90cf-11eb-acc9-ac1f6b738b00 9.09T - - - - - - - ONLINE
gptid/e85bad6d-90cf-11eb-acc9-ac1f6b738b00 9.09T - - - - - - - ONLINE
gptid/6e4220ac-c6b0-11ed-88d6-ac1f6b738b00 9.09T - - - - - - - ONLINE
gptid/e97464f9-90cf-11eb-acc9-ac1f6b738b00 9.09T - - - - - - - ONLINE
gptid/e98fe8b5-90cf-11eb-acc9-ac1f6b738b00 9.09T - - - - - - - ONLINE
gptid/e9d3abb0-90cf-11eb-acc9-ac1f6b738b00 9.09T - - - - - - - ONLINE
gptid/ea54f6eb-90cf-11eb-acc9-ac1f6b738b00 9.09T - - - - - - - ONLINE
special - - - - - - - - -
mirror-2 1.45T 99.4G 1.36T - - 14% 6.67% - ONLINE
da6p1 1.46T - - - - - - - ONLINE
gptid/65cf991f-90d0-11eb-acc9-ac1f6b738b00 1.46T - - - - - - - ONLINE
gptid/6602f626-90d0-11eb-acc9-ac1f6b738b00 1.46T - - - - - - - ONLINE
logs - - - - - - - - -
gptid/b4029892-c0d0-11eb-a3ed-ac1f6b738b00 93.2G 1.65M 93.0G - - 0% 0.00% - ONLINE
cache - - - - - - - - -
gptid/66d6684f-90d0-11eb-acc9-ac1f6b738b00 932G 927G 4.20G - - 0% 99.5% - ONLINE
freenas-boot 58.9G 7.74G 51.1G - - 23% 13% 1.00x ONLINE -
mirror-0 58.9G 7.74G 51.1G - - 23% 13.1% - ONLINE
ada1p2 59.0G - - - - - - - ONLINE
ada0p2 59.0G - - - - - - - ONLINE