Mastodawn

David Cross Mar 7, 2023

ok #freebsd and #zfs mastodon people. Answer me this, I have a ZFS filesystem on machine A, I zfs-send it to machine B (zfs send -R) (zfs receive -Fv). machine A has 4T of space *total*. machine B has 8T of space. when done, machine B has only 250g of free space available), the filesystem is almost _twice_ as large?

Mike Gerdts Mar 7, 2023

@david my first guess is that you have a lot of small files and something is causing zfs to insert a lot of padding.

Is ashift the same on both pools (zpool get ashift, I think)? My guess is the source may be 9 (512 byte minimum block size) and the destination is 12 (4k min block).

Is the source not raidz and destination is raidz?

How are you looking at total space? zpool and zfs commands look at different things?

@mgerdts Not small files, average filesize is close to 1gig (this is a postgres database data filesystem), there are 2 recordsizes on it, a 'precopy' snapshot with 128K records, and then I set it to 8k to get better perfomance and copied everything over, I also set lz4 to zstd on the new copies. no raid on either, straight concat/stripe (underlying hardware does all of the redundancy)

David Cross Mar 7, 2023

@mgerdts I am looking at it via 'zfs list' and 'df' both show compatible information. The main difference seems to be in the refer (I am redoing the receive right now, so I am going from memory), it appears that the receive has multiple full copies.

And the zfs-receive seems to corroborate that by saying it has multiple 'full' streams ... maybe?. In ~7 more hours the receive will be finished

Mike Gerdts Mar 7, 2023

@david maybe the copies or compressratio properties on each dataset will offer clues.

David Cross Mar 7, 2023

@mgerdts ok, so compression ratios are different, by about a factor of 2x.. which explains it. But why? I looked at ashift (zpool property) on both and they are zero on both. Both are zstd (which I additionally forced with a -o on zfs-receive, since ONE of the original ones was lz4.. but even if that was a degenerate compression case in converting lz4 to zstd, it doesn't explain nearly enough of the difference)

David Cross Mar 7, 2023

@mgerdts I did see that checksums are "on" on the source and "skein" on the destination. but on 4t of 8k pages, that's just 16g or 32g of additional space, total (depending on 256bits or 512bits of hashsize, and that's worsecase since that doesn't account for fletcher7 already being 128bits)

Mike Gerdts Mar 7, 2023

@david I'm not sure what to make of ashift=0: that's surely not the real value of ashift. Based on https://openzfs.github.io/openzfs-docs/man/7/zpoolprops.7.html?highlight=ashift saying that ashift can be changed, there have been changes in this area since I last used zfs a lot.

If ashift is the same between the two pools, that points us back to the question of whether you are using raidz or draid and if so, do both pools have the same number of disks per raidz vdev?

zpoolprops.7 — OpenZFS documentation

David Cross Mar 7, 2023

@mgerdts no raidz at all, simple stripe/concat. 4x1T on machine A, 2x4T on machine B.

David Cross Mar 7, 2023

@mgerdts AH-HAH... googling indicates that I need to use zdb vs zpool to get ashift values... and.. there we are. ashift of 12 on the new devices and 9 on the old. I think we have the smoking gun... once I was actually looking in the right place. Thanks!

David Cross Mar 7, 2023

@mgerdts Now all I have to do is kill the pool... and restore.. again.. the ... 5th? time is the charm?

Mike Gerdts Mar 7, 2023

@david 8k recordsize + compression could lead to a poor interaction with ashift=12 as well. Suppose an 8k block would compress to 4200 bytes. With ashift=12, the compressed 8k block will consume 2 x 4k sectors (8k total). With ashift=9, the compressed 8k block will consume 9 x 512b sectors (4.5k total).

With raidz the overhead varies by number of drives in a raidz vdev. See my explanation here:

https://github.com/openzfs/zfs/blob/master/lib/libzfs/libzfs_dataset.c#L5340-L5426

Mike Gerdts Mar 7, 2023

@david re-reading that comment I see that it was updated with draid information, which was integrated after I added this comment with a fix. So things are more complicated with raidz *and* draid. Glad to see the comment update wasn't missed!

Mike Gerdts Mar 8, 2023

@david while we have concluded raidz is not to blame here, I figured it may be worth mentioning that I did a talk on this work while at #Joyent.

Slides: https://us-east.manta.joyent.com/Joyent_Dev/public/docs/2019-06-RAIDZ_on_small_blocks.pdf
Video: https://youtu.be/sTvVIF5v2dw

Contrary to what I predicted back then, today’s NVMe SSDs pretty much all present as 512n, not as 4Kn.

Daniel Carosone Mar 8, 2023

@mgerdts @david

(probably not because it doesn't show up in usage like zfs list -o space, only at the zpool level, but:)

Is there any chance the source has (or had) dedup and that's not been carried across?

David Cross Mar 8, 2023

@uep @mgerdts definitely not on dedupe. the culprit was ashift

Daniel Carosone Mar 8, 2023

@david @mgerdts

ashift and less-optimal compression packing, yeah. Surprised it was that much but still all too plausible

David Cross Mar 8, 2023

@uep @mgerdts I think the key here is that it is a postgres database store, so the recordsize is 8k to align with postgres pagesize, and with ashift of 4k that means BEST case possible compression is 2x, and anything less than 2x is 1x; that means realized compression has to be in the 2.0 to 1.0 range, whereas on the original I was in the 3.x to 4.x range. Math checks out.

Daniel Carosone Mar 9, 2023

@david @mgerdts Yeah, the lower bound on useful compression is a common issue, the upper bound in this case is less obvious but nasty

systemd-jaded.timer Mar 8, 2023

@mgerdts @david wtf, why??