[lkml]   [2011]   [Jul]   [15]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
Messages in this thread
SubjectRe: Mis-Design of Btrfs?
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

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?


 \ /
  Last update: 2011-07-15 16:03    [W:0.081 / U:3.204 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site