Messages in this thread | | | Date | Thu, 12 Jul 2007 15:43:10 -0400 | From | Ric Wheeler <> | Subject | Re: [dm-devel] Re: [RFD] BIO_RW_BARRIER - what it means for devices, filesystems, and dm/md. |
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Valdis.Kletnieks@vt.edu wrote: > 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..)
I think that we are on the same page here - just let the sys admin mount without barriers for big arrays.
1GB of cache, by the way, is really small for some of us ;-)
ric
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