Hi Nikos,
thanks for elaborating on those details.
Hash table collisions, exception store entry commit overhead,
SSD cache flush issues etc. are all valid points relative to performance
and work set footprints in general.
Do you have any performance numbers for your solution vs.
a snapshot one showing the approach is actually superior in
in real configurations?
I'm asking this particularly in the context of your remark
"A write to a not yet hydrated region will be delayed until the
corresponding
region has been hydrated and the hydration of the region starts
immediately."
which'll cause a potentially large working set of delayed writes unless
those
cover the whole eventually larger than 4K region.
How does your 'clone' target perform on such heavy write situations?
In general, performance and storage footprint test results based on the
same set
of read/write tests including heavy loads with region size variations
run on 'clone'
and 'snapshot' would help your point.
Heinz
On 7/10/19 8:45 PM, Nikos Tsironis wrote:
On 7/10/19 12:28 AM, Heinz Mauelshagen wrote:
Hi Nikos,
what is the crucial factor your target offers vs. resynchronizing such a
latency distinct
2-legged mirror with a read-write snapshot (local, fast exception store)
on top, tearing the
mirror down keeping the local leg once fully in sync and merging the
snapshot back into it?
Heinz
Hi Heinz,
The most significant benefits of dm-clone over the solution you propose
is significantly better performance, no need for extra COW space, no
need to merge back a snapshot, and the ability to skip syncing the
unused space of a file system.
1. In order to ensure snapshot consistency, dm-snapshot needs to
commit a completed exception, before signaling the completion of the
write that triggered it to upper layers.
The persistent exception store commits exceptions every time a
metadata area is filled or when there are no more exceptions
in-flight. For a 4K chunk size we have 256 exceptions per metadata
area, so the best case scenario is one commit per 256 writes. Here I
assume a write with size equal to the chunk size of dm-snapshot,
e.g., 4K, so there is no COW overhead, and that we write to new
chunks, so we need to allocate new exceptions.
Part of committing the metadata is flushing the cache of the
underlying device, if there is one. We have seen SSDs which can
sustain hundreds of thousands of random write IOPS, but they take up
to 8ms to flush their cache. In such a case, flushing the SSD cache
every few writes significantly degrades performance.
Moreover, dm-snapshot forces exceptions to complete in the order they
were allocated, to avoid snapshot space leak on crash (commit
230c83afdd9cd). This inserts further latency in exception completions
and thus user write completions.
On the other hand, when cloning a device we don't need to be so
strict and can rely on committing the metadata every time a FLUSH or
FUA bio is written, or periodically, like dm-thin and dm-cache do.
dm-clone does exactly that. When a region/chunk is cloned or
over-written by a write, we just set a bit in the relevant in-core
bitmap. The metadata are committed once every second or when we
receive a FLUSH or FUA bio.
This improves performance significantly and results in increased IOPS
and reduced latency, especially in cases where flushing the disk
cache is very expensive.
2. For large devices, e.g. multi terabyte disks, resynchronizing the
local leg can take a lot of time. If the application running over the
local device is write-heavy, dm-snapshot will end up allocating a
large number of exceptions. This increases the number of hash table
collisions and thus increases the time we need to do a hash table
lookup.
dm-snapshot needs to look up the exception hash tables in order to
service an I/O, so this increases latency and degrades performance.
On the other hand, dm-clone is just testing a bit to see if a region
is cloned or not and decides what to do based on that test.
3. With dm-clone there is no need to reserve extra COW space for
temporarily storing the written data, while the clone device is
syncing. Nor would one need to worry about monitoring and expanding
the COW device to prevent it from filling up.
4. With dm-clone there is no need to merge back potentially several
gigabytes once cloning/syncing completes. We also avoid the relevant
performance degradation incurred by the merging process. Writes just
go directly to the clone device.
5. dm-clone implements support for discards, so it can skip
cloning/syncing the relevant regions. In the case of a large block
device which contains a filesystem with empty space, e.g. a 2TB
device containing 500GB of useful data in a filesystem, this can
significantly reduce the time needed to sync/clone.
This was a rather long email, but I hope it makes the significant
benefits of dm-clone over using dm-snapshot, and our rationale behind
the decision to implement a new target clearer.
I would be more than happy to continue the conversation and focus on any
other questions you may have.
Thanks,
Nikos
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