[LU-20100] writing of many small files below full BRW RPC size results in very bad write speed

• Type: Bug
• Resolution: Unresolved
• Priority: Medium
• Severity: 3
• Assignee: Shaun Tancheff
• Reporter: Shaun Tancheff

Description

writing of many small files below full BRW RPC size results in very bad write speed:
 

When writing sequential files to the system we have noticed a drop in performance when writes files that are smaller than max pages per rpc size. For example running this

for x in {0001..5000}; do echo $x; dd bs=1M count=5 if=/dev/zero of=file$x; done

performance will be ~1G till the 143-145th file, then drop off only to recover later.
On the other hand running this

for x in {0001..5000}; do echo $x; dd bs=1M count=16 if=/dev/zero of=file$x; done

will result in no performance drop.
max_pages_per_rpc=4096
page size 4k

performance drop means "singe digit MB/s" instead of > 1GB/s, i.e. the slowdown is ~ 100x.