linux-kernel - Re: [dm-devel] Re: [RFD] BIO_RW_BARRIER - what it means for devices, filesystems, and dm/md.

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Date:	Thu, 12 Jul 2007 15:43:10 -0400
From:	Ric Wheeler <ric@....com>
To:	Valdis.Kletnieks@...edu
CC:	Tejun Heo <htejun@...il.com>, david@...g.hm,
	Stefan Bader <Stefan.Bader@...ibm.com>,
	Phillip Susi <psusi@....rr.com>,
	device-mapper development <dm-devel@...hat.com>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-raid@...r.kernel.org, Jens Axboe <jens.axboe@...cle.com>,
	David Chinner <dgc@....com>,
	Andreas Dilger <adilger@...sterfs.com>
Subject: Re: [dm-devel] Re: [RFD] BIO_RW_BARRIER - what it means for devices,
 filesystems, and dm/md.



Valdis.Kletnieks@...edu wrote:
> On Wed, 11 Jul 2007 18:44:21 EDT, Ric Wheeler said:
>> Valdis.Kletnieks@...edu wrote:
>>> On Tue, 10 Jul 2007 14:39:41 EDT, Ric Wheeler said:
>>>
>>>> All of the high end arrays have non-volatile cache (read, on power loss, it is a 
>>>> promise that it will get all of your data out to permanent storage). You don't 
>>>> need to ask this kind of array to drain the cache. In fact, it might just ignore 
>>>> you if you send it that kind of request ;-)
>>> OK, I'll bite - how does the kernel know whether the other end of that
>>> fiberchannel cable is attached to a DMX-3 or to some no-name product that
>>> may not have the same assurances?  Is there a "I'm a high-end array" bit
>>> in the sense data that I'm unaware of?
>>>
>> There are ways to query devices (think of hdparm -I in S-ATA/P-ATA drives, SCSI 
>> has similar queries) to see what kind of device you are talking to. I am not
>> sure it is worth the trouble to do any automatic detection/handling of this.
>>
>> In this specific case, it is more a case of when you attach a high end (or 
>> mid-tier) device to a server, you should configure it without barriers for its
>> exported LUNs.
> 
> I don't have a problem with the sysadmin *telling* the system "the other end of
> that fiber cable has characteristics X, Y and Z".  What worried me was that it
> looked like conflating "device reported writeback cache" with "device actually
> has enough battery/hamster/whatever backup to flush everything on a power loss".
> (My back-of-envelope calculation shows for a worst-case of needing a 1ms seek
> for each 4K block, a 1G cache can take up to 4 1/2 minutes to sync.  That's
> a lot of battery..)

I think that we are on the same page here - just let the sys admin mount without 
barriers for big arrays.

1GB of cache, by the way, is really small for some of us ;-)

ric

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