On 2017/7/12 上午4:10, Eric Wheeler wrote:
> On Tue, 11 Jul 2017, Coly Li wrote:
>
>> When gfs2 does metadata read ahead, currently flags (REQ_RAHEAD | REQ_META)
>> are used to submit bio. Flag REQ_META is just a hint for block trace, not
>> for block layer code to handle a bio as metadata request.
>>
>> When doing read ahead for metadata, A REQ_PRIO flag on the metadata bio
>> is very informative to block layer code. For example, if bcache is used as
>> a I/O cache for gfs2, it will be possible for bcache code to cache the
>> pre-fetched metadata blocks on cache device as well, which may be
>> probably to improve metadata I/O performance if the following requests
>> hit the cache.
>>
>> This patch adds REQ_PRIO flag when submitting a metadata readahead bio.
>> A meta data read ahead bio may come from I/O requests for bitmap,
>> directoriesmeta or other general metadata request.
>>
>
> Are there any places in gfs2 where REQ_PRIO should be placed on
> latency-sensitive metadata writes? They would then writeback in bcache
> after the relevant bcache patch is merged.
Hi Eric,
I just grep REQ_META in fs/gfs2, there are 10 locations use REQ_META.
Among the 10 locations, REQ_PRIO shows up at 3 locations, another 3
locations are REQ_READAHEAD and I add REQ_PRIO to them, the rested
locations are,
1) log.c:662
In log_write_header(), used to flush gfs2 log header. The author does
not add REQ_PRIO on purpose, because I see REQ_FUA is there. And for no
barrier condition, REQ_PRIO is added with REQ_FUA removed. So it's well
done here.
2) meta_io.c:455
Here gfs2_meta_ra() tries to read first metadata block, cache the
metadata block in bcache should be helpful.
3) ops_fstype.c:250
This is for gfs2_read_sb(), it is only called once by init_sb(), we
don't need to cache this block by bcache.
4) quota.c:733
This is in gfs2_write_buf_to_page() when gfs2 disk quota is updated,
we can have it in cache device.
So it seems REQ_PRIO might be added at meta_io.c:455, and quota.c:733 to
provide more hint to bcache.
Thanks for suggestion.
Coly
>> Signed-off-by: Coly Li <colyli@xxxxxxx>
>> ---
>> fs/gfs2/bmap.c | 5 +++--
>> fs/gfs2/dir.c | 4 +++-
>> fs/gfs2/meta_io.c | 4 +++-
>> 3 files changed, 9 insertions(+), 4 deletions(-)
>>
>> diff --git a/fs/gfs2/bmap.c b/fs/gfs2/bmap.c
>> index 9fa3aef9a5b3..fa3ea29f39cf 100644
>> --- a/fs/gfs2/bmap.c
>> +++ b/fs/gfs2/bmap.c
>> @@ -291,8 +291,9 @@ static void gfs2_metapath_ra(struct gfs2_glock *gl,
>> if (trylock_buffer(rabh)) {
>> if (!buffer_uptodate(rabh)) {
>> rabh->b_end_io = end_buffer_read_sync;
>> - submit_bh(REQ_OP_READ, REQ_RAHEAD | REQ_META,
>> - rabh);
>> + submit_bh(REQ_OP_READ,
>> + REQ_RAHEAD | REQ_META | REQ_PRIO,
>> + rabh);
>> continue;
>> }
>> unlock_buffer(rabh);
>> diff --git a/fs/gfs2/dir.c b/fs/gfs2/dir.c
>> index db427658ccd9..0741e4018f8c 100644
>> --- a/fs/gfs2/dir.c
>> +++ b/fs/gfs2/dir.c
>> @@ -1514,7 +1514,9 @@ static void gfs2_dir_readahead(struct inode *inode, unsigned hsize, u32 index,
>> continue;
>> }
>> bh->b_end_io = end_buffer_read_sync;
>> - submit_bh(REQ_OP_READ, REQ_RAHEAD | REQ_META, bh);
>> + submit_bh(REQ_OP_READ,
>> + REQ_RAHEAD | REQ_META | REQ_PRIO,
>> + bh);
>> continue;
>> }
>> brelse(bh);
>> diff --git a/fs/gfs2/meta_io.c b/fs/gfs2/meta_io.c
>> index fabe1614f879..6103d1c816ef 100644
>> --- a/fs/gfs2/meta_io.c
>> +++ b/fs/gfs2/meta_io.c
>> @@ -461,7 +461,9 @@ struct buffer_head *gfs2_meta_ra(struct gfs2_glock *gl, u64 dblock, u32 extlen)
>> bh = gfs2_getbuf(gl, dblock, CREATE);
>>
>> if (!buffer_uptodate(bh) && !buffer_locked(bh))
>> - ll_rw_block(REQ_OP_READ, REQ_RAHEAD | REQ_META, 1, &bh);
>> + ll_rw_block(REQ_OP_READ,
>> + REQ_RAHEAD | REQ_META | REQ_PRIO,
>> + 1, &bh);
>> brelse(bh);
>> dblock++;
>> extlen--;
>> --
>> 2.12.0
>>
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--
Coly Li
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