From mboxrd@z Thu Jan 1 00:00:00 1970 From: Chris Mason Subject: Re: Mis-Design of Btrfs? Date: Fri, 15 Jul 2011 09:20:00 -0400 Message-ID: <1310735821-sup-2589@shiny> References: <20110623105337.GD3753@ethz.ch> <20110627164637.377314e2@notabene.brown> <4E0AF091.9030301@redhat.com> <20110714155620.6e9ac2cc@notabene.brown> <4E1E866E.2050405@redhat.com> <20110714163836.35a729c1@notabene.brown> <4E1E9372.3030208@redhat.com> <1310696606-sup-710@shiny> <20110715163354.74fa2083@notabene.brown> <1310728784-sup-9024@shiny> <4E20395C.1000301@redhat.com> Content-Type: text/plain; charset=UTF-8 Cc: NeilBrown , david , Nico Schottelius , LKML , linux-btrfs , Alasdair G Kergon To: Ric Wheeler Return-path: In-reply-to: <4E20395C.1000301@redhat.com> List-ID: Excerpts from Ric Wheeler's message of 2011-07-15 08:58:04 -0400: > On 07/15/2011 12:34 PM, Chris Mason wrote: [ triggering IO retries on failed crc or other checks ] > > > > But, maybe the whole btrfs model is backwards for a generic layer. > > Instead of sending down ios and testing when they come back, we could > > just set a verification function (or stack of them?). > > > > For metadata, btrfs compares the crc and a few other fields of the > > metadata block, so we can easily add a compare function pointer and a > > void * to pass in. > > > > The problem is the crc can take a lot of CPU, so btrfs kicks it off to > > threading pools so saturate all the cpus on the box. But there's no > > reason we can't make that available lower down. > > > > If we pushed the verification down, the retries could bubble up the > > stack instead of the other way around. > > > > -chris > > I do like the idea of having the ability to do the verification and retries down > the stack where you actually have the most context to figure out what is possible... > > Why would you need to bubble back up anything other than an error when all > retries have failed? By bubble up I mean that if you have multiple layers capable of doing retries, the lowest levels would retry first. Basically by the time we get an -EIO_ALREADY_RETRIED we know there's nothing that lower level can do to help. -chris