Current large bucket (size >16MB) layout is a preparation for the future
zoned cache device, by which the cache device bucket can be aligned to
zone size and boundary of the zoned SSD.
Commit ffa470327572 ("bcache: add bucket_size_hi into struct
cache_sb_disk for large bucket") extends bucket size from 16MB to 1TB,
but this change is problematic and can be improved,
- The bucket size is always power-of-2 value, it is unnecessary to add
extra member in struct cache_sb_disk to store the real size value.
Just using existing bucket_size to store the order of power-of-2 value
is enough.
- The added bucket_size_hi is behind d[SB_JOURNAL_BUCKETS] in struct
cache_sb_disk, which breaks a very implicit restriction from macro
csum_set(). This restriction requires member d[] must be the last
member in data structure to calculate the checksum.
This series is to improve the above issue from bcache kernel code. The
basic ideas of this series are,
- Add BCH_FEATURE_INCOMPAT_LOG_LARGE_BUCKET_SIZE incompat feature bit.
When it is set, for bucket size >16MB, bucket_size stores the order
of power-of-2 bucket size value. Then no extra space introduced and
csum_set() has all data to calculate super block checksum.
- Rename struct cache_sb_disk's bucket_size_hi to obso_bucket_size_hi,
and rename the incompat feature bit BCH_FEATURE_INCOMPAT_LARGE_BUCKET
to BCH_FEATURE_INCOMPAT_OBSO_LARGE_BUCKET, to indicate they are not
used anymore and obsoleted.
- If a cache device was created for bucket size >16MB with obsoleted
layout, bcache driver will print error message for the obsoleted
layout and all following attached bcache device will be read-only. If
there is dirty data on cache device, after the writeback accomplished
the cache device can be re-created with new bucket_size usage by the
latest bcache-tools.
Because the btree node usage is not optimized, I doubt there is people
really uses bucket size >16MB at this moment, except me. But this is
something needs to be improved and better as soon as possible.
Coly Li
---
Coly Li (4):
bcache: fix typo from SUUP to SUPP in features.h
bcache: check unsupported feature sets for bcache register
bcache: introduce BCH_FEATURE_INCOMPAT_LOG_LARGE_BUCKET_SIZE for large
bucket
bcache: set bcache device into read-only mode for
BCH_FEATURE_INCOMPAT_OBSO_LARGE_BUCKET
drivers/md/bcache/features.c | 2 +-
drivers/md/bcache/features.h | 30 +++++++++++++++++----
drivers/md/bcache/super.c | 51 +++++++++++++++++++++++++++++++++---
include/uapi/linux/bcache.h | 2 +-
4 files changed, 75 insertions(+), 10 deletions(-)
--
2.26.2
