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Merge pull request #51370 from anthonyeleven/anthonyeleven/correct-space-amp

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doc/rados/configuration: correct space amp in bluestore-config-ref.rst
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Anthony D'Atri 2023-05-06 11:28:53 -04:00 committed by GitHub
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doc/rados/configuration/bluestore-config-ref.rst
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@ -431,7 +431,7 @@ GateWay (RGW) deployments that host large numbers of small files
For example, when an RGW client stores a 1KB S3 object, it is written to a
single RADOS object. With the default :confval:`min_alloc_size` value, 4KB of
underlying drive space is allocated. This means that roughly
(4KB - 1KB) == 3KB is allocated but never used, which corresponds to 300%
(4KB - 1KB) == 3KB of that RADOS object's allocated space is never used, which corresponds to 300%
overhead or 25% efficiency. Similarly, a 5KB user object will be stored
as one 4KB and one 1KB RADOS object, again stranding 4KB of device capacity,
though in this case the overhead is a much smaller percentage. Think of this
@ -440,8 +440,8 @@ thus decreases rapidly as user object size increases.
An easily missed additional subtlety is that this
takes place for *each* replica. So when using the default three copies of
data (3R), a 1KB S3 object actually consumes roughly 9KB of storage device
capacity. If erasure coding (EC) is used instead of replication, the
data (3R), a 1KB S3 object actually consumes 12KB of storage device
capacity, with 11KB of overhead. If erasure coding (EC) is used instead of replication, the
amplification may be even higher: for a ``k=4,m=2`` pool, our 1KB S3 object
will allocate (6 * 4KB) = 24KB of device capacity.