From: Zdenek Kabelac <zkabelac@redhat.com>
To: linux-lvm@redhat.com, Gionatan Danti <g.danti@assyoma.it>
Subject: Re: [linux-lvm] Snapshot behavior on classic LVM vs ThinLVM
Date: Mon, 15 May 2017 14:50:52 +0200 [thread overview]
Message-ID: <b3372535-379f-8771-7395-6b0e4ba72f0d@redhat.com> (raw)
In-Reply-To: <b0e4f2d1cfca0ea32da27b1cb34bf1be@assyoma.it>
Dne 14.5.2017 v 22:39 Gionatan Danti napsal(a):
> Il 12-05-2017 15:42 Joe Thornber ha scritto:
>> On Fri, May 12, 2017 at 03:02:58PM +0200, Gionatan Danti wrote:
>>> On 02/05/2017 13:00, Gionatan Danti wrote:
>>> >>Anyway, I think (and maybe I am wrong...) that the better solution is to
>>> >>fail *all* writes to a full pool, even the ones directed to allocated
>>> >>space. This will effectively "freeze" the pool and avoid any
>>> >>long-standing inconsistencies.
>>
>> I think dm-thin behaviour is fine given the semantics of write
>> and flush IOs.
>>
>> A block device can complete a write even if it hasn't hit the physical
>> media, a flush request needs to come in at a later time which means
>> 'flush all IOs that you've previously completed'. So any software using
>> a block device (fs, database etc), tends to generate batches of writes,
>> followed by a flush to commit the changes. For example if there was a
>> power failure between the batch of write io completing and the flush
>> completing you do not know how much of the writes will be visible when
>> the machine comes back.
>>
>> When a pool is full it will allow writes to provisioned areas of a thin to
>> succeed. But if any writes failed due to inability to provision then a
>> REQ_FLUSH io to that thin device will *not* succeed.
>>
>> - Joe
>
> True, but the real problem is that most of the failed flushes will *not* bring
> the filesystem read-only, as both ext4 and xfs seems to go read-only only when
> *metadata* updates fail. As this very same list recommend using ext4 with
> errors=remount-ro on the basis that putting the filesystem in a read-only
> state after any error I the right thing, I was somewhat alarmed to find that,
> as far I can tell, ext4 goes read-only on metadata errors only.
>
> So, let me reiterate: can we consider thinlvm + xfs as safe as thinlvm + ext4
> + errors=remount-ro?
Hi
I still think you are mixing apples & oranges together and you expecting
answer '42' :)
There is simply NO simple answer. Every case has its pros & cons.
There is simply cases where XFS beats Ext4 and there are opposite situations
as well.
Also you WILL always get WRITE error - if your application doesn't care about
write error - why do you expect any block-device logic could rescue you ??
Out-of-space thin-pool is simply a device which looks like seriously damaged
disk where you always read something without any problem and you fail to write
things here and there.
IMHO both filesystem XFS & Ext4 on recent kernels do work well - but no one
can say there are no problems at all.
Things are getting better - but planning usage of thin-pool to 'recover'
overfilled pool is simple BAD planning. You should plan your thin-pool usage
to NOT run out-of-space.
And last comment I always say - full thin-pool it not similar to full
filesystem where you drop some 'large' file and you are happily working again
- it's not working this way - and if someone hoped into this - he needs to use
something else ATM.
Regards
Zdenek
next prev parent reply other threads:[~2017-05-15 12:50 UTC|newest]
Thread overview: 94+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-04-06 14:31 [linux-lvm] Snapshot behavior on classic LVM vs ThinLVM Gionatan Danti
2017-04-07 8:19 ` Mark Mielke
2017-04-07 9:12 ` Gionatan Danti
2017-04-07 13:50 ` L A Walsh
2017-04-07 16:33 ` Gionatan Danti
2017-04-13 12:59 ` Stuart Gathman
2017-04-13 13:52 ` Xen
2017-04-13 14:33 ` Zdenek Kabelac
2017-04-13 14:47 ` Xen
2017-04-13 15:29 ` Stuart Gathman
2017-04-13 15:43 ` Xen
2017-04-13 17:26 ` Stuart D. Gathman
2017-04-13 17:32 ` Stuart D. Gathman
2017-04-14 15:17 ` Xen
2017-04-14 7:27 ` Gionatan Danti
2017-04-14 7:23 ` Gionatan Danti
2017-04-14 15:23 ` Xen
2017-04-14 15:53 ` Gionatan Danti
2017-04-14 16:08 ` Stuart Gathman
2017-04-14 17:36 ` Xen
2017-04-14 18:59 ` Gionatan Danti
2017-04-14 19:20 ` Xen
2017-04-15 8:27 ` Xen
2017-04-15 23:35 ` Xen
2017-04-17 12:33 ` Xen
2017-04-15 21:22 ` Xen
2017-04-15 21:49 ` Xen
2017-04-15 21:48 ` Xen
2017-04-18 10:17 ` Zdenek Kabelac
2017-04-18 13:23 ` Gionatan Danti
2017-04-18 14:32 ` Stuart D. Gathman
2017-04-19 7:22 ` Xen
2017-04-07 22:24 ` Mark Mielke
2017-04-08 11:56 ` Gionatan Danti
2017-04-07 18:21 ` Tomas Dalebjörk
2017-04-13 10:20 ` Gionatan Danti
2017-04-13 12:41 ` Xen
2017-04-14 7:20 ` Gionatan Danti
2017-04-14 8:24 ` Zdenek Kabelac
2017-04-14 9:07 ` Gionatan Danti
2017-04-14 9:37 ` Zdenek Kabelac
2017-04-14 9:55 ` Gionatan Danti
2017-04-22 7:14 ` Gionatan Danti
2017-04-22 16:32 ` Xen
2017-04-22 20:58 ` Gionatan Danti
2017-04-22 21:17 ` Zdenek Kabelac
2017-04-23 5:29 ` Xen
2017-04-23 9:26 ` Zdenek Kabelac
2017-04-24 21:02 ` Xen
2017-04-24 21:59 ` Zdenek Kabelac
2017-04-26 7:26 ` Gionatan Danti
2017-04-26 7:42 ` Zdenek Kabelac
2017-04-26 8:10 ` Gionatan Danti
2017-04-26 11:23 ` Zdenek Kabelac
2017-04-26 13:37 ` Gionatan Danti
2017-04-26 14:33 ` Zdenek Kabelac
2017-04-26 16:37 ` Gionatan Danti
2017-04-26 18:32 ` Stuart Gathman
2017-04-26 19:24 ` Stuart Gathman
2017-05-02 11:00 ` Gionatan Danti
2017-05-12 13:02 ` Gionatan Danti
2017-05-12 13:42 ` Joe Thornber
2017-05-14 20:39 ` Gionatan Danti
2017-05-15 12:50 ` Zdenek Kabelac [this message]
2017-05-15 14:48 ` Gionatan Danti
2017-05-15 15:33 ` Zdenek Kabelac
2017-05-16 7:53 ` Gionatan Danti
2017-05-16 10:54 ` Zdenek Kabelac
2017-05-16 13:38 ` Gionatan Danti
2018-02-27 18:39 ` Xen
2018-02-28 9:26 ` Zdenek Kabelac
2018-02-28 19:07 ` Gionatan Danti
2018-02-28 21:43 ` Zdenek Kabelac
2018-03-01 7:14 ` Gionatan Danti
2018-03-01 8:31 ` Zdenek Kabelac
2018-03-01 9:43 ` Gianluca Cecchi
2018-03-01 11:10 ` Zdenek Kabelac
2018-03-01 9:52 ` Gionatan Danti
2018-03-01 11:23 ` Zdenek Kabelac
2018-03-01 12:48 ` Gionatan Danti
2018-03-01 16:00 ` Zdenek Kabelac
2018-03-01 16:26 ` Gionatan Danti
2018-03-03 18:32 ` Xen
2018-03-04 20:34 ` Zdenek Kabelac
2018-03-03 18:17 ` Xen
2018-03-04 20:53 ` Zdenek Kabelac
2018-03-05 9:42 ` Gionatan Danti
2018-03-05 10:18 ` Zdenek Kabelac
2018-03-05 14:27 ` Gionatan Danti
2018-03-03 17:52 ` Xen
2018-03-04 23:27 ` Zdenek Kabelac
2017-04-22 21:22 ` Zdenek Kabelac
2017-04-24 13:49 ` Gionatan Danti
2017-04-24 14:48 ` Zdenek Kabelac
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