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

Valdis.Kletnieks@vt.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.

ric
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