Hi all,
We’re using cache-tier with write-back mode but the write throughput is not as
good as we expect. We use CephFS and create a 20GB file in it. While data is
writing, we use iostat to get the disk statistics. From iostat, we saw that ssd
(cache-tier) is idle most of the time and hdd (storage-tier) is busy all the
time. From the document
“When admins configure tiers with writeback mode, Ceph clients write data to
the cache tier and receive an ACK from the cache tier. In time, the data
written to the cache tier migrates to the storage tier and gets flushed from
the cache tier.”
So the data is write to cache-tier and then flush to storage tier when dirty
ratio is more than 0.4? The word “in time” in the document confused me.
We found that the throughput of creating a new file is slower than overwrite an
existing file, and ssd has more write when doing overwrite. We then look into
the source code and log. A newly created file goes to proxy_write, which is
followed by a promote_object. Does this means that the object actually goes to
storage pool directly and then be promoted to the cache-tier when creating a
new file?
Thanks,
Ting Yi Lin
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