From: Gionatan Danti <g.danti@assyoma.it>
To: Zhiyong Ye <yezhiyong@bytedance.com>
Cc: LVM general discussion and development <linux-lvm@redhat.com>
Subject: Re: [linux-lvm] Why is the performance of my lvmthin snapshot so poor
Date: Tue, 14 Jun 2022 16:54:28 +0200 [thread overview]
Message-ID: <d927c03eef10c508161ad52daa7e908e@assyoma.it> (raw)
In-Reply-To: <5970db8d-462f-0e35-741c-fa0fdc188fa2@bytedance.com>
Il 2022-06-14 15:29 Zhiyong Ye ha scritto:
> The reason for this may be that when the volume creates a snapshot,
> each write to an existing block will cause a COW (Copy-on-write), and
> the COW is a copy of the entire data block in chunksize, for example,
> when the chunksize is 64k, even if only 4k of data is written, the
> entire 64k data block will be copied. I'm not sure if I understand
> this correctly.
Yes, in your case, the added copies are lowering total available IOPs.
But note how the decrease is sub-linear (from 64K to 1M you have a 16x
increase in chunk size but "only" a 10x hit in IOPs): this is due to the
lowered metadata overhead.
A last try: if you can, please regenerate your thin volume with 64K
chunks and set fio to execute 64K requests. Lets see if LVM is at least
smart enough to avoid coping a to-be-completely-overwritten chunks.
Regards.
--
Danti Gionatan
Supporto Tecnico
Assyoma S.r.l. - www.assyoma.it
email: g.danti@assyoma.it - info@assyoma.it
GPG public key ID: FF5F32A8
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next prev parent reply other threads:[~2022-06-14 14:54 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-06-13 8:49 [linux-lvm] Why is the performance of my lvmthin snapshot so poor Zhiyong Ye
2022-06-14 7:04 ` Gionatan Danti
2022-06-14 10:16 ` Zhiyong Ye
2022-06-14 12:56 ` Gionatan Danti
2022-06-14 13:29 ` Zhiyong Ye
2022-06-14 14:54 ` Gionatan Danti [this message]
2022-06-15 7:42 ` Zhiyong Ye
2022-06-15 9:34 ` Gionatan Danti
2022-06-15 9:46 ` Zhiyong Ye
2022-06-15 12:40 ` Gionatan Danti
2022-06-15 16:39 ` Demi Marie Obenour
2022-06-16 7:53 ` Demi Marie Obenour
2022-06-16 13:22 ` Gionatan Danti
2022-06-16 16:19 ` Demi Marie Obenour
2022-06-16 19:50 ` Gionatan Danti
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