linux-kernel - Re: [PATCHSET block#for-2.6.36-post] block: replace barrier with sequenced flush

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Date:	Mon, 23 Aug 2010 11:19:13 -0400
From:	Ric Wheeler <rwheeler@...hat.com>
To:	Jens Axboe <jaxboe@...ionio.com>
CC:	Christoph Hellwig <hch@....de>, Tejun Heo <tj@...nel.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	"linux-scsi@...r.kernel.org" <linux-scsi@...r.kernel.org>,
	"linux-ide@...r.kernel.org" <linux-ide@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-raid@...r.kernel.org" <linux-raid@...r.kernel.org>,
	"James.Bottomley@...e.de" <James.Bottomley@...e.de>,
	"tytso@....edu" <tytso@....edu>,
	"chris.mason@...cle.com" <chris.mason@...cle.com>,
	"swhiteho@...hat.com" <swhiteho@...hat.com>,
	"konishi.ryusuke@....ntt.co.jp" <konishi.ryusuke@....ntt.co.jp>,
	"dm-devel@...hat.com" <dm-devel@...hat.com>,
	"vst@...b.net" <vst@...b.net>, "jack@...e.cz" <jack@...e.cz>,
	"hare@...e.de" <hare@...e.de>
Subject: Re: [PATCHSET block#for-2.6.36-post] block: replace barrier with
  sequenced flush

On 08/23/2010 10:01 AM, Jens Axboe wrote:
> On 2010-08-23 15:58, Ric Wheeler wrote:
>> On 08/23/2010 08:48 AM, Christoph Hellwig wrote:
>>> On Mon, Aug 23, 2010 at 02:30:33PM +0200, Tejun Heo wrote:
>>>> It might be useful to give several example configurations with
>>>> different cache configurations.  I don't have much experience with
>>>> battery backed arrays but aren't they suppose to report write through
>>>> cache automatically?
>>>
>>> They usually do.  I have one that doesn't, but SYNCHRONIZE CACHE on
>>> it is so fast that it effectively must be a no-op.
>>>
>>
>> Arrays are not a problem in general - they normally have internally, redundant
>> batteries to hold up the cache.
>>
>> The issue is when you have an internal hardware RAID card with a large cache.
>> Those cards sit in your server and the batteries on the card protect its
>> internal cache, but do not have the capacity to hold up the drives behind it.
>>
>> Normally, those drives should have their write cache disabled, but sometimes
>> (especially with S-ATA disks) this is not done.
>
> The problem purely exists on arrays that report write back cache enabled
> AND don't implement SYNC_CACHE as a noop. Do any of them exist, or are
> they purely urban legend?
>


Hi Jens,

There are actually two distinct problems:

(1) arrays with a non-volatile write cache (battery backed, navram, whatever) 
that do not NOOP a SYNC_CACHE command. I know of one brand that seems to do 
this, but it is not a common brand. If we do not issue flushes for write through 
caches, I think that we will avoid this in any case.

(2) hardware raid cards with internal buffer memory and on-card battery backup 
(they sit in your server, disks sit in jbod like expansion shelves). These are 
fine if the drives in those shelves have write cache disabled.

ric
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