Re: zvol performance expecations? zvol swap?

[Sad Clouds <cryintothebluesky%gmail.com@localhost>]

On Wed, 17 Feb 2021 19:04:34 -0500
Greg Troxel <gdt%lexort.com@localhost> wrote:

> 
> Sad Clouds <cryintothebluesky%gmail.com@localhost> writes:
> 
> > On Wed, 17 Feb 2021 13:15:39 -0500
> > Greg Troxel <gdt%lexort.com@localhost> wrote:
> >
> >> 
> >> Suppose I create a 16G zvol on a pool that is a disklabel
> >> partition on an SSD.   I would expect read/write performance that
> >> is near the native SSD read/write speed.
> >
> > Why would you expect that? In other words, you're expecting that a
> > complex file system like ZFS would have near zero overheads, which
> > is not possible. When you take into considerations things like
> > checksums, compression, encryption, etc, then the overheads could
> > be quite significant.
> 
> I have not enabled compression or encryption, and I forgot about
> checksums.
> 
> However, the use case for zvol is having zfs being a logical volume
> manager, and it would seem reasonably feasible to get high
> efficiencies via allocation of large numbers of blocks at once.   So
> 10-20% slower wouldn't surprise me.
> 
> Have you used zvols?  What was your experience?

Not used it on NetBSD. Used a bit on Solaris and Linux at home, but
never really cared about throughput performance. My personal feeling
about ZFS - it's designed to provide good data integrity, but a bit
complicated and has too many features/options. I prefer simpler designs.

With regard to zvols, the following is normally recommended for Linux
swap:

zfs create -V 8G -b $(getconf PAGESIZE) \
-o logbias=throughput -o sync=always -o checksum=off \
-o compression=off -o dedup=off -o primarycache=metadata \
-o com.sun:auto-snapshot=false <pool>/swap

Also, several tuning options for "zfs create":
atime=off      - Disable atime updates.
recordsize=16K - For small files and random I/O.
recordsize=1M  - For large files and sequential I/O.
xattr=sa       - Linux only, store xattr directly in inodes.