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..)
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