Il 2022-06-16 09:53 Demi Marie Obenour ha scritto:
That seems reasonable. My conclusion is that dm-thin (which is what
LVM
uses) is not a good fit for workloads with a lot of small random writes
and frequent snapshots, due to the 64k minimum chunk size. This also
explains why dm-thin does not allow smaller blocks: not only would it
only support very small thin pools, it would also have massive metadata
write overhead. Hopefully dm-thin v2 will improve the situation.
I think that, in this case, no free lunch really exists. I tried the
following thin provisioning methods, each with its strong & weak points:
lvmthin: probably the more flexible of the mainline kernel options. You
pay for r/m/w only when allocating a small block (say 4K) the first time
after taking a snapshot. It is fast and well integrated with lvm command
line. Con: bad behavior on out-of-space condition
xfs + reflink: a great, simple to use tool when applicable. It has a
very small granularity (4K) with no r/m/w. Cons: requires fine tuning
for good performance when reflinking big files; IO freezes during
metadata copy for reflink; a very small granularity means sequential IO
is going to suffer heavily (see here for more details:
https://marc.info/?l=linux-xfs&m=157891132109888&w=2)
btrfs: very small granularity (4K) and many integrated features. Cons:
bad performance overall, especially when using mechanical HDD
vdo: is provides small granularity (4K) thin provisioning, compression
and deduplication. Cons: (still) out-of-tree; requires a powerloss
protected writeback cache to maintain good performance; no snapshot
capability
zfs: designed for the ground up for pervasive CoW, with many features
and ARC/L2ARC. Cons: out-of-tree; using small granularity (4K) means bad
overall performance; using big granularity (128K by default) is a
necessary compromise for most HDD pools.
For what it is worth, I settled on ZFS when using out-of-tree modules is
not an issue and lvmthin otherwise (but I plan to use xfs + reflink more
in the future).
Do you have any information to share about dm-thin v2? I heard about it
some years ago, but I found no recent info.
Regards.
--
Danti Gionatan
Supporto Tecnico
Assyoma S.r.l. - www.assyoma.it
email: g.danti@xxxxxxxxxx - info@xxxxxxxxxx
GPG public key ID: FF5F32A8
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