Messages in this thread | | | Date | Fri, 15 Jul 2011 15:07:24 +0100 | From | Hugo Mills <> | Subject | Re: Mis-Design of Btrfs? |
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On Fri, Jul 15, 2011 at 10:00:35AM -0400, Chris Mason wrote: > Excerpts from Ric Wheeler's message of 2011-07-15 09:31:37 -0400: > > On 07/15/2011 02:20 PM, Chris Mason wrote: > > > 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 > > > > Absolutely sounds like the most sane way to go to me, thanks! > > > > It really seemed like a good idea, but I just realized it doesn't work > well when parts of the stack transform the data. > > Picture dm-crypt on top of raid1. If raid1 is responsible for the > crc retries, there's no way to crc the data because it needs to be > decrypted first. > > I think the raided dm-crypt config is much more common (and interesting) > than multiple layers that can retry for other reasons (raid1 on top of > raid10?)
Isn't this a case where the transformative mid-layer would replace the validation function before passing it down the stack? So btrfs hands dm-crypt a checksum function; dm-crypt then stores that function for its own purposes and hands off a new function to the DM layer below that which decrypts the data and calls the btrfs checksum function it stored earlier.
> In other words, do we really want to do a lot of design work for > multiple layers where each one maintains multiple copies of the data > blocks? Are there configs where this really makes sense?
Hugo.
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