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Re: [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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