On Thu, Feb 08, 2018 at 07:52:05PM +0100, Goffredo Baroncelli wrote:
> On 02/06/2018 12:15 AM, Liu Bo wrote:
> [...]
> > One way to mitigate the data loss pain is to expose 'bad chunks',
> > i.e. degraded chunks, to users, so that they can use 'btrfs balance'
> > to relocate the whole chunk and get the full raid6 protection again
> > (if the relocation works).
>
> [...]
> [...]
>
> > +
> > + /* read lock please */
> > + do {
> > + seq = read_seqbegin(&fs_info->bc_lock);
> > + list_for_each_entry(bc, &fs_info->bad_chunks, list) {
> > + len += snprintf(buf + len, PAGE_SIZE - len, "%llu\n",
> > + bc->chunk_offset);
> > + /* chunk offset is u64 */
> > + if (len >= PAGE_SIZE)
> > + break;
> > + }
> > + } while (read_seqretry(&fs_info->bc_lock, seq));
>
> Using this interface, how many chunks can you list ? If I read the code
> correctly, only up to fill a kernel page.
>
> If my math are correctly (PAGE_SIZE=4k, a u64 could require up to 19 bytes)
> it is possible to list only few hundred of chunks (~200). Not more; and the
> last one could be even listed incomplete.
>
That's true.
> IIRC a chunk size is max 1GB; If you lost a 500GB of disks, the chunks to
> list could be more than 200.
>
> My first suggestion is to limit the number of chunks to show to 200 (a page
> should be big enough to contains all these chunks offset). If the chunks
> number are greater, ends the list with a marker (something like '[...]\n').
> This would solve the ambiguity about the fact that the list chunks are
> complete or not. Anyway you cannot list all the chunks.
>
Good point, and I need to add one more field to each record to specify
it's metadata or data.
So what I have in my mind is that this kind of error is rare and
reflects bad sectors on disk, but if there are really that many
errors, I think we need to replace the disk without hesitation.
> However, my second suggestions is to ... change completely the interface.
> What about adding a directory in sysfs, where each entry is a chunk ?
>
> Something like:
>
> /sys/fs/btrfs/<FS-UUID>/chunks/<chunks-offset>/type #
> data/metadata/sys
> /sys/fs/btrfs/<FS-UUID>/chunks/<chunks-offset>/profile #
> dup/linear....
> /sys/fs/btrfs/<FS-UUID>/chunks/<chunks-offset>/size # size
> /sys/fs/btrfs/<FS-UUID>/chunks/<chunks-offset>/devs # chunks devs
>
> And so on.
>
> Checking "[...]<chunks-offset>/devs", it would be easy understand if the
> chunk is in "degraded" mode or not.
I'm afraid we cannot do that as it'll occupy too much memory for large
filesystems given a typical chunk size is 1GB.
>
> However I have to admit that I don't know how feasible is iterate over a
> sysfs directory which is a map of a kernel objects list.
>
> I think that if these interface would be good enough, we could get rid of a
> lot of ioctl(TREE_SEARCH) from btrfs-progs.
>
TREE_SEARCH is faster than iterating sysfs (if we could), isn't it?
thanks,
-liubo
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