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From: Tomasz Chmielewski <tch@virtall.com>
To: "linux-btrfs@vger.kernel.org" <linux-btrfs@vger.kernel.org>
Subject: Re: ditto blocks on ZFS
Date: Thu, 22 May 2014 16:28:44 +0100	[thread overview]
Message-ID: <20140522162844.268cbed6@s9> (raw)

> I thought an important idea behind btrfs was that we avoid by design
> in the first place the very long and vulnerable RAID rebuild scenarios
> suffered for block-level RAID...

This may be true for SSD disks - for ordinary disks it's not entirely
the case.

For most RAID rebuilds, it still seems way faster with software RAID-1
where one drive is being read at its (almost) full speed, and the other
is being written to at its (almost) full speed (assuming no other IO
load).

With btrfs RAID-1, the way balance is made after disk replace, it takes
lots of disk head movements resulting in overall small speed to rebuild
the RAID, especially with lots of snapshots and related fragmentation.

And the balance is still not smart and is causing reads from one device,
and writes to *both* devices (extra unnecessary write to the
healthy device - while it should read from the healthy device and write
to the replaced device only).


Of course, other factors such as the amount of data or disk IO usage
during rebuild apply.


-- 
Tomasz Chmielewski
http://wpkg.org

             reply	other threads:[~2014-05-22 15:28 UTC|newest]

Thread overview: 18+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2014-05-22 15:28 Tomasz Chmielewski [this message]
  -- strict thread matches above, loose matches on Subject: below --
2014-05-16  3:07 ditto blocks on ZFS Russell Coker
2014-05-17 12:50 ` Martin
2014-05-17 14:24   ` Hugo Mills
2014-05-18 16:09   ` Russell Coker
2014-05-19 20:36     ` Martin
2014-05-19 21:47       ` Brendan Hide
2014-05-20  2:07         ` Russell Coker
2014-05-20 14:07           ` Austin S Hemmelgarn
2014-05-20 20:11             ` Brendan Hide
2014-05-20 14:56           ` ashford
2014-05-21  2:51             ` Russell Coker
2014-05-21 23:05               ` Martin
2014-05-22 11:10                 ` Austin S Hemmelgarn
2014-05-22 22:09               ` ashford
2014-05-23  3:54                 ` Russell Coker
2014-05-23  8:03                   ` Duncan
2014-05-21 23:29           ` Konstantinos Skarlatos

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