From: Goffredo Baroncelli <kreijack@inwind.it>
To: Nicholas D Steeves <nsteeves@gmail.com>, linux-btrfs@vger.kernel.org
Subject: Re: dstat shows unexpected result for two disk RAID1
Date: Wed, 9 Mar 2016 21:50:05 +0100 [thread overview]
Message-ID: <56E08C7D.3040304@inwind.it> (raw)
In-Reply-To: <CAD=QJKg6xFSFjJdDyD+v0Rd2afeB-ZRRy2SncC1Db5fyT3cf9A@mail.gmail.com>
On 2016-03-09 21:25, Nicholas D Steeves wrote:
> grr. Gmail is terrible :-/
>
> I understood that a btrfs RAID1 would at best grab one block from sdb
> and then one block from sdd in round-robin fashion, or at worse grab
> one chunk from sdb and then one chunk from sdd. Alternatively I
> thought that it might read from both simultaneously, to make sure that
> all data matches, while at the same time providing single-disk
> performance. None of these was the case. Running a single
> IO-intensive process reads from a single drive.
>
> Did I misunderstand the documentation and is this normal, or is this a bug?
> Nicholas
In a case of a BTRFS RAID, I knew that a process read from a drive depending by its pid.
I don't know if it is changed. But what from you write it seems that it still true today.
>
> On 9 March 2016 at 15:21, Nicholas D Steeves <nsteeves@gmail.com> wrote:
>> Hello everyone,
>>
>> I've run into an expected behaviour for a my two disk RAID1. I mount
>> with UUIDs, because sometimes my USB disk gets /dev/sdc instead of
>> /dev/sdd. The two elements of my RAID1 are currently sdb and sdd.
>>
>> dstat -tdD total,sdb,sdc,sdd
>>
>> It seems that per process, reads come from either sdb or sdd. This
>> surprises me, because I understood that a btrfs RAID1
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next prev parent reply other threads:[~2016-03-09 20:50 UTC|newest]
Thread overview: 18+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-03-09 20:21 dstat shows unexpected result for two disk RAID1 Nicholas D Steeves
2016-03-09 20:25 ` Nicholas D Steeves
2016-03-09 20:50 ` Goffredo Baroncelli [this message]
2016-03-09 21:26 ` Chris Murphy
2016-03-09 22:51 ` Nicholas D Steeves
2016-03-11 23:42 ` Nicholas D Steeves
2016-03-09 21:36 ` Roman Mamedov
2016-03-09 21:43 ` Chris Murphy
2016-03-09 22:08 ` Nicholas D Steeves
2016-03-10 4:06 ` Duncan
2016-03-10 5:01 ` Chris Murphy
2016-03-10 8:10 ` Duncan
2016-03-12 0:04 ` Nicholas D Steeves
2016-03-12 0:10 ` Nicholas D Steeves
2016-03-12 1:20 ` Chris Murphy
2016-04-06 3:58 ` Nicholas D Steeves
2016-04-06 12:02 ` Austin S. Hemmelgarn
2016-04-22 22:36 ` Nicholas D Steeves
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