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

[Valdis.Kletnieks@vt.edu]

On Wed, 11 Jul 2007 18:44:21 EDT, Ric Wheeler said:
> 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.

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..)

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