Re: [zfs] recordsize: unexpected increase of disk usage when increasing it
Date: Tue, 18 Jan 2022 14:51:37 UTC
2.1M /workspace/test1M/1 - Rich On Tue, Jan 18, 2022 at 9:47 AM Alan Somers <asomers@freebsd.org> wrote: > Yeah, it does. Just check "du -sh <FILENAME>". zdb there is showing > you the logical size of the record, but it isn't showing how many disk > blocks are actually allocated. > > On Tue, Jan 18, 2022 at 7:30 AM Rich <rincebrain@gmail.com> wrote: > > > > Really? I didn't know it would still trim the tails on files with > compression off. > > > > ... > > > > size 1179648 > > parent 34 > > links 1 > > pflags 40800000004 > > Indirect blocks: > > 0 L1 DVA[0]=<3:c02b96c000:1000> > DVA[1]=<3:c810733000:1000> [L1 ZFS plain file] skein lz4 unencrypted LE > contiguous unique double size=20000L/1000P birth=35675472L/35675472P fill=2 > cksum=5cfba24b351a09aa:8bd9dfef87c5b625:906ed5c3252943db:bed77ce51ad540d4 > > 0 L0 DVA[0]=<2:a0827db4000:100000> [L0 ZFS plain file] > skein uncompressed unencrypted LE contiguous unique single > size=100000L/100000P birth=35675472L/35675472P fill=1 > cksum=95b06edf60e5f54c:af6f6950775d0863:8fc28b0783fcd9d3:2e44676e48a59360 > > 100000 L0 DVA[0]=<2:a0827eb4000:100000> [L0 ZFS plain file] > skein uncompressed unencrypted LE contiguous unique single > size=100000L/100000P birth=35675472L/35675472P fill=1 > cksum=62a1f05769528648:8197c8a05ca9f1fb:a750c690124dd2e0:390bddc4314cd4c3 > > > > It seems not? > > > > - Rich > > > > > > On Tue, Jan 18, 2022 at 9:23 AM Alan Somers <asomers@freebsd.org> wrote: > >> > >> On Tue, Jan 18, 2022 at 7:13 AM Rich <rincebrain@gmail.com> wrote: > >> > > >> > Compression would have made your life better here, and possibly also > made it clearer what's going on. > >> > > >> > All records in a file are going to be the same size pre-compression - > so if you set the recordsize to 1M and save a 131.1M file, it's going to > take up 132M on disk before compression/raidz overhead/whatnot. > >> > >> Not true. ZFS will trim the file's tails even without compression > enabled. > >> > >> > > >> > Usually compression saves you from the tail padding actually > requiring allocation on disk, which is one reason I encourage everyone to > at least use lz4 (or, if you absolutely cannot for some reason, I guess zle > should also work for this one case...) > >> > > >> > But I would say it's probably the sum of last record padding across > the whole dataset, if you don't have compression on. > >> > > >> > - Rich > >> > > >> > On Tue, Jan 18, 2022 at 8:57 AM Florent Rivoire <florent@rivoire.fr> > wrote: > >> >> > >> >> TLDR: I rsync-ed the same data twice: once with 128K recordsize and > >> >> once with 1M, and the allocated size on disk is ~3% bigger with 1M. > >> >> Why not smaller ? > >> >> > >> >> > >> >> Hello, > >> >> > >> >> I would like some help to understand how the disk usage evolves when > I > >> >> change the recordsize. > >> >> > >> >> I've read several articles/presentations/forums about recordsize in > >> >> ZFS, and if I try to summarize, I mainly understood that: > >> >> - recordsize is the "maximum" size of "objects" (so "logical blocks") > >> >> that zfs will create for both -data & metadata, then each object is > >> >> compressed and allocated to one vdev, splitted into smaller (ashift > >> >> size) "physical" blocks and written on disks > >> >> - increasing recordsize is usually good when storing large files that > >> >> are not modified, because it limits the nb of metadata objects > >> >> (block-pointers), which has a positive effect on performance > >> >> - decreasing recordsize is useful for "databases-like" workloads (ie: > >> >> small random writes inside existing objects), because it avoids write > >> >> amplification (read-modify-write a large object for a small update) > >> >> > >> >> Today, I'm trying to observe the effect of increasing recordsize for > >> >> *my* data (because I'm also considering defining special_small_blocks > >> >> & using SSDs as "special", but not tested nor discussed here, just > >> >> recordsize). > >> >> So, I'm doing some benchmarks on my "documents" dataset (details in > >> >> "notes" below), but the results are really strange to me. > >> >> > >> >> When I rsync the same data to a freshly-recreated zpool: > >> >> A) with recordsize=128K : 226G allocated on disk > >> >> B) with recordsize=1M : 232G allocated on disk => bigger than 128K > ?!? > >> >> > >> >> I would clearly expect the other way around, because bigger > recordsize > >> >> generates less metadata so smaller disk usage, and there shouldn't be > >> >> any overhead because 1M is just a maximum and not a forced size to > >> >> allocate for every object. > >> > >> A common misconception. The 1M recordsize applies to every newly > >> created object, and every object must use the same size for all of its > >> records (except possibly the last one). But objects created before > >> you changed the recsize will retain their old recsize, file tails have > >> a flexible recsize. > >> > >> >> I don't mind the increased usage (I can live with a few GB more), but > >> >> I would like to understand why it happens. > >> > >> You might be seeing the effects of sparsity. ZFS is smart enough not > >> to store file holes (and if any kind of compression is enabled, it > >> will find long runs of zeroes and turn them into holes). If your data > >> contains any holes that are >= 128 kB but < 1MB, then they can be > >> stored as holes with a 128 kB recsize but must be stored as long runs > >> of zeros with a 1MB recsize. > >> > >> However, I would suggest that you don't bother. With a 128kB recsize, > >> ZFS has something like a 1000:1 ratio of data:metadata. In other > >> words, increasing your recsize can save you at most 0.1% of disk > >> space. Basically, it doesn't matter. What it _does_ matter for is > >> the tradeoff between write amplification and RAM usage. 1000:1 is > >> comparable to the disk:ram of many computers. And performance is more > >> sensitive to metadata access times than data access times. So > >> increasing your recsize can help you keep a greater fraction of your > >> metadata in ARC. OTOH, as you remarked increasing your recsize will > >> also increase write amplification. > >> > >> So to summarize: > >> * Adjust compression settings to save disk space. > >> * Adjust recsize to save RAM. > >> > >> -Alan > >> > >> >> > >> >> I tried to give all the details of my tests below. > >> >> Did I do something wrong ? Can you explain the increase ? > >> >> > >> >> Thanks ! > >> >> > >> >> > >> >> > >> >> =============================================== > >> >> A) 128K > >> >> ========== > >> >> > >> >> # zpool destroy bench > >> >> # zpool create -o ashift=12 bench > >> >> /dev/gptid/3c0f5cbc-b0ce-11ea-ab91-c8cbb8cc3ad4 > >> >> > >> >> # rsync -av --exclude '.zfs' /mnt/tank/docs-florent/ /bench > >> >> [...] > >> >> sent 241,042,476,154 bytes received 353,838 bytes 81,806,492.45 > bytes/sec > >> >> total size is 240,982,439,038 speedup is 1.00 > >> >> > >> >> # zfs get recordsize bench > >> >> NAME PROPERTY VALUE SOURCE > >> >> bench recordsize 128K default > >> >> > >> >> # zpool list -v bench > >> >> NAME SIZE ALLOC FREE > >> >> CKPOINT EXPANDSZ FRAG CAP DEDUP HEALTH ALTROOT > >> >> bench 2.72T 226G 2.50T > >> >> - - 0% 8% 1.00x ONLINE - > >> >> gptid/3c0f5cbc-b0ce-11ea-ab91-c8cbb8cc3ad4 2.72T 226G 2.50T > >> >> - - 0% 8.10% - ONLINE > >> >> > >> >> # zfs list bench > >> >> NAME USED AVAIL REFER MOUNTPOINT > >> >> bench 226G 2.41T 226G /bench > >> >> > >> >> # zfs get all bench |egrep "(used|referenced|written)" > >> >> bench used 226G - > >> >> bench referenced 226G - > >> >> bench usedbysnapshots 0B - > >> >> bench usedbydataset 226G - > >> >> bench usedbychildren 1.80M - > >> >> bench usedbyrefreservation 0B - > >> >> bench written 226G - > >> >> bench logicalused 226G - > >> >> bench logicalreferenced 226G - > >> >> > >> >> # zdb -Lbbbs bench > zpool-bench-rcd128K.zdb > >> >> > >> >> > >> >> > >> >> =============================================== > >> >> B) 1M > >> >> ========== > >> >> > >> >> # zpool destroy bench > >> >> # zpool create -o ashift=12 bench > >> >> /dev/gptid/3c0f5cbc-b0ce-11ea-ab91-c8cbb8cc3ad4 > >> >> # zfs set recordsize=1M bench > >> >> > >> >> # rsync -av --exclude '.zfs' /mnt/tank/docs-florent/ /bench > >> >> [...] > >> >> sent 241,042,476,154 bytes received 353,830 bytes 80,173,899.88 > bytes/sec > >> >> total size is 240,982,439,038 speedup is 1.00 > >> >> > >> >> # zfs get recordsize bench > >> >> NAME PROPERTY VALUE SOURCE > >> >> bench recordsize 1M local > >> >> > >> >> # zpool list -v bench > >> >> NAME SIZE ALLOC FREE > >> >> CKPOINT EXPANDSZ FRAG CAP DEDUP HEALTH ALTROOT > >> >> bench 2.72T 232G 2.49T > >> >> - - 0% 8% 1.00x ONLINE - > >> >> gptid/3c0f5cbc-b0ce-11ea-ab91-c8cbb8cc3ad4 2.72T 232G 2.49T > >> >> - - 0% 8.32% - ONLINE > >> >> > >> >> # zfs list bench > >> >> NAME USED AVAIL REFER MOUNTPOINT > >> >> bench 232G 2.41T 232G /bench > >> >> > >> >> # zfs get all bench |egrep "(used|referenced|written)" > >> >> bench used 232G - > >> >> bench referenced 232G - > >> >> bench usedbysnapshots 0B - > >> >> bench usedbydataset 232G - > >> >> bench usedbychildren 1.96M - > >> >> bench usedbyrefreservation 0B - > >> >> bench written 232G - > >> >> bench logicalused 232G - > >> >> bench logicalreferenced 232G - > >> >> > >> >> # zdb -Lbbbs bench > zpool-bench-rcd1M.zdb > >> >> > >> >> > >> >> > >> >> =============================================== > >> >> Notes: > >> >> ========== > >> >> > >> >> - the source dataset contains ~50% of pictures (raw files and jpg), > >> >> and also some music, various archived documents, zip, videos > >> >> - no change on the source dataset while testing (cf size logged by > resync) > >> >> - I repeated the tests twice (128K, then 1M, then 128K, then 1M), and > >> >> same results > >> >> - probably not important here, but: > >> >> /dev/gptid/3c0f5cbc-b0ce-11ea-ab91-c8cbb8cc3ad4 is a Red 3TB CMR > >> >> (WD30EFRX), and /mnt/tank/docs-florent/ is a 128K-recordsize dataset > >> >> on another zpool that I never tweaked except ashit=12 (because using > >> >> the same model of Red 3TB) > >> >> > >> >> # zfs --version > >> >> zfs-2.0.6-1 > >> >> zfs-kmod-v2021120100-zfs_a8c7652 > >> >> > >> >> # uname -a > >> >> FreeBSD xxxxxxxxx 12.2-RELEASE-p11 FreeBSD 12.2-RELEASE-p11 > >> >> 75566f060d4(HEAD) TRUENAS amd64 >