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

[Guy Watkins]

} -----Original Message-----
} From: linux-raid-owner@xxxxxxxxxxxxxxx [mailto:linux-raid-
} owner@xxxxxxxxxxxxxxx] On Behalf Of Valdis.Kletnieks@xxxxxx
} Sent: Thursday, July 12, 2007 1:35 PM
} To: ric@xxxxxxx
} Cc: Tejun Heo; david@xxxxxxx; Stefan Bader; Phillip Susi; device-mapper
} development; linux-fsdevel@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx;
} linux-raid@xxxxxxxxxxxxxxx; Jens Axboe; David Chinner; Andreas Dilger
} Subject: Re: [dm-devel] Re: [RFD] BIO_RW_BARRIER - what it means for
} devices, filesystems, and dm/md.
} 
} On Wed, 11 Jul 2007 18:44:21 EDT, Ric Wheeler said:
} > Valdis.Kletnieks@xxxxxx 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..)

Most hardware RAID devices I know of use the battery to save the cache while
the power is off.  When the power is restored it flushes the cache to disk.
If the power failure lasts longer than the batteries then the cache data is
lost, but the batteries last 24+ hours I beleve.

A big EMC array we had had enough battery power to power about 400 disks
while the 16 Gig of cache was flushed.  I think EMC told me the batteries
would last about 20 minutes.  I don't recall if the array was usable during
the 20 minutes.  We never tested a power failure.

Guy

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