Re: RAID creation resync behaviors

From: Shaohua Li <shli@kernel.org>
To: David Brown <david.brown@hesbynett.no>
Cc: linux-raid@vger.kernel.org, jes.sorensen@gmail.com, neilb@suse.de
Subject: Re: RAID creation resync behaviors
Date: Wed, 3 May 2017 18:54:54 -0700	[thread overview]
Message-ID: <20170504015454.d4obiuume6e3yrdv@kernel.org> (raw)
In-Reply-To: <fe3c30e4-b0dd-639e-7db7-344aa93ce198@hesbynett.no>

On Wed, May 03, 2017 at 11:06:01PM +0200, David Brown wrote:
> On 03/05/17 22:27, Shaohua Li wrote:
> > Hi,
> > 
> > Currently we have different resync behaviors in array creation.
> > 
> > - raid1: copy data from disk 0 to disk 1 (overwrite)
> > - raid10: read both disks, compare and write if there is difference (compare-write)
> > - raid4/5: read first n-1 disks, calculate parity and then write parity to the last disk (overwrite)
> > - raid6: read all disks, calculate parity and compare, and write if there is difference (compare-write)
> > 
> > Write whole disk is very unfriendly for SSD, because it reduces lifetime. And
> > if user already does a trim before creation, the unncessary write could make
> > SSD slower in the future. Could we prefer compare-write to overwrite if mdadm
> > detects the disks are SSD? Surely sometimes compare-write is slower than
> > overwrite, so maybe add new option in mdadm. An option to let mdadm trim SSD
> > before creation sounds reasonable too.
> > 
> 
> When doing the first sync, md tracks how far its sync has got, keeping a
> record in the metadata in case it has to be restarted (such as due to a
> reboot while syncing).  Why not simply /not/ sync stripes until you first
> write to them?  It may be that a counter of synced stripes is not enough,
> and you need a bitmap (like the write intent bitmap), but it would reduce
> the creation sync time to 0 and avoid any writes at all.

For raid 4/5/6, this means we always must do a full stripe write for any normal
write if it hits a range not synced. This would harm the performance of the
norma write. For raid1/10, this sounds more appealing. But since each bit in
the bitmap will stand for a range. If only part of the range is written by
normal IO, we have two choices. sync the range immediately and clear the bit,
this sync will impact normal IO. Don't do the sync immediately, but since the
bit is set (which means the range isn't synced), read IO can only access the
first disk, which is harmful too.

Thanks,
Shaohua