From: Mike Snitzer <snitzer@redhat.com>
To: Nir Soffer <nsoffer@redhat.com>
Cc: David Teigland <teigland@redhat.com>,
Vojtech Juranek <vjuranek@redhat.com>,
Denis Chaplygin <dchaplyg@redhat.com>,
linux-lvm@redhat.com
Subject: Re: [linux-lvm] Mixing devices with different logical or physical block size in oVirt LVM based storage
Date: Mon, 4 Feb 2019 11:25:27 -0500 [thread overview]
Message-ID: <20190204162527.GA2896@redhat.com> (raw)
In-Reply-To: <CAMRbyyv5qcsqmmP0uk+hEBmZJfZ-stV7XWUH23eJDnNMZYs7QA@mail.gmail.com>
On Sat, Feb 02 2019 at 6:54pm -0500,
Nir Soffer <nsoffer@redhat.com> wrote:
> We working on enabling 4k block size in oVirt block storage domain, built
> using VG
> on multipath devices on shared storage.
> We have incomplete support for 4k, added in 2011, for this bug:
> [1]https://bugzilla.redhat.com/732980
> When creating or extending a VG, we check that all PVs are using same
> logical and
> phyisical block size, and we store both logical and physical block size in
> the VG tags.
> We get the block sizes from
> /sys/block/dm-X/queue/{logical,physical}_block_size.
> We also enforce that device physical block size is not smaller than
> logical block size,
> This check was added in this patch, trying to enable block size != 512.
> There is no
> explanation in the patch or in the review comments why we need to validate
> this.
>
> [2]https://github.com/oVirt/vdsm/commit/7e79153705891a91a06eb31cd642fb209d10ff86
> When we start to use a VG, we validate that all the devices are using the
> stored logical
> and physical block size.
> In vdsm itself, we use the logical block size to manage vdsm metadata,
> assuming that writing
> and reading one block of logical block size bytes is atomic, and we can
> read and write
> different blocks from different hosts at the same time.
> The relevant code validating PV block sizes is here:
>
> [3]https://github.com/oVirt/vdsm/blob/8b043e402f41d8a82b9f832be5f582b8520b38bc/lib/vdsm/storage/lvm.py#L1110
> Reading the comments in bug 732980, I don't see anything about physical
> block size. It looks
> like this is unnecessary check, and we should check only the logical block
> size.
> Regarding mixing devices with different logical block size, according to
> [4]https://bugzilla.redhat.com/show_bug.cgi?id=732980#c8
> We should not extend an LV over devices with different block size, as this
> will change the device
> logical block size (e.g change from 512 to 4k), and the change may break
> the upper layer that
> already use the device and assume the previous logical block size.
This idea that 4K writes to a 512b physical drive aren't going to be
atomic, and that that is going to be the basis for some upper level
failure is handwaving and overly paranoid TBH.
> Based on this, I think we are ok with limiting VG to devices with same
> logical block size, so any
> LV can be extended to any device.
> I think this code should change to:
> 1. When creating a VG, check that all PVs use the same logical block size
> 2. Store the logical block size in the VG tag
> 3. When extending the VG, check that the new PVs use the same logical
> block size
> 4. When starting to use a VG, check that stored logical block size matches
> PVs logical block size
> What do you think?
I think you shouldn't care. Or please show me a case where all this
concern matters.
Mike
next prev parent reply other threads:[~2019-02-04 16:25 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-02-02 23:54 [linux-lvm] Mixing devices with different logical or physical block size in oVirt LVM based storage Nir Soffer
2019-02-04 16:25 ` Mike Snitzer [this message]
2019-02-13 9:14 ` Vojtech Juranek
2019-02-13 20:39 ` Mike Snitzer
2019-02-13 21:41 ` Nir Soffer
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20190204162527.GA2896@redhat.com \
--to=snitzer@redhat.com \
--cc=dchaplyg@redhat.com \
--cc=linux-lvm@redhat.com \
--cc=nsoffer@redhat.com \
--cc=teigland@redhat.com \
--cc=vjuranek@redhat.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).