Re: XFS on top RAID10 with odd drives count and 2 near copies

On 14/02/2012 08:31, CoolCold wrote:

On Tue, Feb 14, 2012 at 3:02 AM,[off-list ref]  wrote:

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On Tue, Feb 14, 2012 at 01:40:25AM +0400, CoolCold wrote:

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On Mon, Feb 13, 2012 at 4:09 PM, Stan
Hoeppner[off-list ref]  wrote:

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On 2/12/2012 2:16 PM, CoolCold wrote:

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First of all, Stan, thanks for such detailed answer, I
greatly appreciate this!

You're welcome. ?You may or may not appreciate this reply. ?It
got really long. ?I tried to better explain the XFS+md linear
array setup.

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There are several reasons for this - 1) I've made decision to
use LMV for all "data" volumes (those are except /, /boot,
/home , etc) ?2) there will be mysql database which will need
backups with snapshots 3)

So you need LVM for snaps, got it.

Well, I do not think LVM gives you snaps. I think you need to close
down the mysql database to have a consistent DB, then make backup,
then reactivate mysql. I may be wrong, tho.

You are a bit wrong here. MySQL in general supports two storage
types - MyISAM&  InnoDB. While InnoDB is ACID transactional engine,
MyISAM isn't. So, one should be able to backup InnoDB with snapshots
without interrupting workload and it will do recovery/transaction
rollbackup on startup. For MyISAM engine, snapshots will produce
unpredictable results as partial update may happen. But, snapshots
are useful in any case, because they allow to do backup in 4 steps:
1) "FLUSH TABLES WITH READ LOCK" - this will flush all buffers and
close databases 2) lvcreate -s&&  unlock tables.... - doing snapshot
of data and releasing lock 3) copying it somewhere 4) lvremove .... -
releasing snapshot

So, in such situation, work will stop for 1)&  2) , not cosuming time
for 3) .

Very roughly speaking, taking an LVM snapshot is like pulling the plug on the system - if the database engine is able to recover reliably from a power fail (by replaying logs, or whatever), then it can restore data copied by a snapshot.  I don't know MySQL well enough to say if it can do such recovery.

I believe that if you have XFS on LVM, then making a snapshot will first "freeze" the filesystem, take the snapshot, then "thaw" the filesystem.  This process will sync the system, flushing out outstanding writes, and delay new writes until the "thaw" - thus you get a bit better than a "power-off copy".  In particular, you don't get zeroed-out files no matter what you've done with your barrier settings.

I think ext4 also freezes in the same way, but only with later kernels.

mvh.,

David