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

[Ric Wheeler <ric emc com>]

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