From: Zdenek Kabelac <zkabelac@redhat.com>
To: LVM general discussion and development <linux-lvm@redhat.com>,
Ilia Zykov <mail@izyk.ru>
Subject: Re: [linux-lvm] exposing snapshot block device
Date: Wed, 23 Oct 2019 15:05:14 +0200 [thread overview]
Message-ID: <8a27504b-1b3a-7126-7ade-18f57b819ead@redhat.com> (raw)
In-Reply-To: <e282b0b3-e373-c454-8243-3b09c23dd2a1@izyk.ru>
Dne 23. 10. 19 v 14:20 Ilia Zykov napsal(a):
> On 23.10.2019 14:08, Gionatan Danti wrote:
>>
>> For example, consider a completely filled 64k chunk thin volume (with
>> thinpool having ample free space). Snapshotting it and writing a 4k
>> block on origin will obviously cause a read of the original 64k chunk,
>> an in-memory change of the 4k block and a write of the entire modified
>> 64k block to a new location. But writing, say, a 1 MB block should *not*
>> cause the same read on source: after all, the read data will be
>> immediately discarded, overwritten by the changed 1 MB block.
>>
>> However, my testing shows that source chunks are always read, even when
>> completely overwritten.
>
> Not only read but sometimes write.
> I watched it without snapshot. Only zeroing was enabled. Before wrote
> new chunks "dd bs=1048576 ..." chunks were zeroed. But for security it's
> good. IMHO: In this case good choice firstly write chunks to the disk
> and then give this chunks to the volume.
Yep - we are recommending to disable zeroing as soon as chunksize >512K.
But for 'security' reason the option it's up to users to select what fits the
needs in the best way - there is no 'one solution fits them all' in this case.
Clearly when you put a modern filesystem (ext4, xfs...) on top of thinLV - you
can't read junk data - filesystem knows very well about written portions.
But if you will access thinLV device on 'block-level' with 'dd' command you
might see some old data trash if zeroing is disabled...
For smaller chunksizes zeroing is usually not a big deal - with bigger chunks
it slows down initial provisioning in major way - but once the block is
provisioned there are no further costs....
Regards
Zdenek
next prev parent reply other threads:[~2019-10-23 13:05 UTC|newest]
Thread overview: 53+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-10-22 10:47 [linux-lvm] exposing snapshot block device Dalebjörk, Tomas
2019-10-22 13:57 ` Zdenek Kabelac
2019-10-22 15:29 ` Dalebjörk, Tomas
2019-10-22 15:36 ` Zdenek Kabelac
2019-10-22 16:13 ` Dalebjörk, Tomas
2019-10-23 10:26 ` Zdenek Kabelac
2019-10-23 10:56 ` Tomas Dalebjörk
2019-10-22 16:15 ` Stuart D. Gathman
2019-10-22 17:02 ` Tomas Dalebjörk
2019-10-22 21:38 ` Gionatan Danti
2019-10-22 22:53 ` Stuart D. Gathman
2019-10-23 6:58 ` Gionatan Danti
2019-10-23 10:06 ` Tomas Dalebjörk
2019-10-23 10:12 ` Zdenek Kabelac
2019-10-23 10:46 ` Zdenek Kabelac
2019-10-23 11:08 ` Gionatan Danti
2019-10-23 11:24 ` Tomas Dalebjörk
2019-10-23 11:26 ` Tomas Dalebjörk
2019-10-24 16:01 ` Zdenek Kabelac
2019-10-25 16:31 ` Tomas Dalebjörk
2019-11-04 5:54 ` Tomas Dalebjörk
2019-11-04 10:07 ` Zdenek Kabelac
2019-11-04 14:40 ` Tomas Dalebjörk
2019-11-04 15:04 ` Zdenek Kabelac
2019-11-04 17:28 ` Tomas Dalebjörk
2019-11-05 16:24 ` Zdenek Kabelac
2019-11-05 16:40 ` Mikulas Patocka
2019-11-05 20:56 ` Tomas Dalebjörk
2019-11-06 9:22 ` Zdenek Kabelac
2019-11-07 16:54 ` Mikulas Patocka
2019-11-07 17:29 ` Tomas Dalebjörk
2020-09-04 12:09 ` Tomas Dalebjörk
2020-09-04 12:37 ` Zdenek Kabelac
2020-09-07 13:09 ` Mikulas Patocka
2020-09-07 14:14 ` Dalebjörk, Tomas
2020-09-07 14:17 ` Zdenek Kabelac
2020-09-07 16:34 ` Tomas Dalebjörk
2020-09-07 16:42 ` Zdenek Kabelac
2020-09-07 17:37 ` Tomas Dalebjörk
2020-09-07 17:50 ` Zdenek Kabelac
2020-09-08 12:32 ` Dalebjörk, Tomas
2020-09-07 19:56 ` Tomas Dalebjörk
2020-09-07 20:22 ` Tomas Dalebjörk
2020-09-07 21:02 ` Tomas Dalebjörk
2019-10-23 12:12 ` Ilia Zykov
2019-10-23 12:20 ` Ilia Zykov
2019-10-23 13:05 ` Zdenek Kabelac [this message]
2019-10-23 14:40 ` Gionatan Danti
2019-10-23 15:46 ` Ilia Zykov
2019-10-23 12:59 ` Zdenek Kabelac
2019-10-23 14:37 ` Gionatan Danti
2019-10-23 15:37 ` Zdenek Kabelac
2019-10-23 17:16 ` Gionatan Danti
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