Re: Possible bug? bcache: error opening /dev/md126: Not enough buckets

[Jason Warr <jason-/cow75dQlsI@public.gmane.org>]

On 06/14/2013 12:34 AM, John Clark wrote:
> Hi there,
>
> I'm new to kernel mailing lists so beg your pardon if I'm asking this
> question in the wrong place or in the wrong way.
>
> Anyway, I'm tantalisingly close to getting my bcache-enabled system
> up-and-running, but I've hit a brick wall which seems might be a bug
> or undocumented performance limit of some kind. I'm hoping Kent or
> others on this fine list can help.
>
> I have successfully created the following two RAID1 block devices, as
> shown per /proc/mdstat:
>
> Personalities : [raid1]
> md126 : active raid1 sdb[0] sdc[1]
>       1953383360 blocks super 1.2 [2/2] [UU]
>
> md127 : active raid1 sdd[0] sde[1]
>       117155200 blocks super 1.2 [2/2] [UU]
>
> /dev/md126 comprises two 2.0TB spinning HDD's which I'm intending to
> use as a bcache backing device
> /dev/md127 comprises two 60GB SSD's which I'm intending to use as a
> bcache cache device
>
> So far so good.
>
> Creating the bcache superblock on the md127 appears to go smoothly:
>
> root@bigdata:~# make-bcache -C /dev/md127
> UUID:                   1736b20f-6b85-4fee-a801-4cf7c1bba009
> Set UUID:               b2c9e8e2-0606-4d51-bf9a-e9b8a6f150b3
> version:                0
> nbuckets:               228818
> block_size:             8
> bucket_size:            1024
> nr_in_set:              1
> nr_this_dev:            0
> first_bucket:           1
>
>
> Creating the bcache superblock on md126 *appears* to succeed also:
>
> root@bigdata:~# make-bcache -B /dev/md/spinning
>
> UUID:                   9e2bd59a-a413-4ad2-a07b-6998dfa3e049
> Set UUID:               8c70baad-6941-4550-9fc8-b009e016b00d
> version:                1
> block_size:             8
> data_offset:            16
>
> However, the following is output on syslog when executing the above command:
> Jun 14 14:21:37 bigdata kernel: [ 1602.102646] bcache: error opening
> /dev/md126: Not enough buckets
>
> Indeed, although I can register the cache device (and the UUID shows
> up in /sys/fs/bcache), all attempts to register the backing device
> fails as follows:
>
> root@bigdata:~# echo /dev/md126 >/sys/fs/bcache/register
> -bash: echo: write error: Invalid argument
>
> And, sure enough, the backing device UUID doesn't appear in
> /sys/fs/bcache nor at /dev/bcacheN
>
> I've tried using the make-bcache -b parameter to specify a different
> bucket size but I still get the same failure (unless I choose
> ridiculously high or low bucket sizes, which results in bucket size
> errors to be emitted on syslog).
>
> As I was just finishing up writing this and was about to hit SEND, I
> noticed something that I wish I had noticed earlier - specifically
> that the "bcache-3.2" section of the bcache git repo was last updated
> 6 months ago.
>
> The kernel I'm running is built from that tree. I had assumed that it
> was the version 3.2 kernel patched with a relatively current version
> of bcache, but now I think I may be seriously mistaken and my problems
> could be due to the possibility that I'm running an old and buggy
> version of bcache. I don't want to get ahead of myself, but it seems a
> decent guess that my problems might be stemming from the fact that the
> 'bcache-3.2' tree uses old version of bcache.
>
> At the risk of getting seriously ahead of myself, assuming that my
> problem is due to 'bcache-3.2' being out of date and buggy, I'll ask
> this: what is the recommended way to get a current and stable version
> of bcache running on a stable linux kernel? I'm wanting to use bcache
> on a production system and so I'm a little wary of building the
> 'bcache-for-3.10' tree. Ideally, I'd like to use bcache on a 3.2
> kernel, because that's the kernel version readily available presently
> in debian stable/wheezy so I will be less likely to encounter kernel
> version incompatibilities with my debian wheezy system.
>
> Many thanks in advance if anyone can help me along here.
>
> John
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Build from the 3.10 RC series.  It may not be "stable" yet but should be
within a few weeks I think and it has the bcache bits in it.

I have been running from the bcache-for3.10 tree on CentOS 6 for a few
months without any issues.  I will be using the mainline 3.10-RC5 tree
sometime over the weekend.