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From: Christian Ebner <c.ebner@proxmox.com>
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Subject: Re: [pbs-devel] [PATCH v4 proxmox-backup 5/5] fix #5331: garbage
collection: avoid multiple chunk atime updates
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On 3/25/25 12:56, Thomas Lamprecht wrote:
> Am 21.03.25 um 10:32 schrieb Christian Ebner:
>> To reduce the number of atimes updates, keep track of the recently
>> marked chunks in phase 1 of garbage to avoid multiple atime updates
>> via expensive utimensat() calls.
>>
>> Recently touched chunks are tracked by storing the chunk digests in
>> an LRU cache of fixed capacity. By inserting a digest, the chunk will
>> be the most recently touched one and if already present in the cache
>> before insert, the atime update can be skipped.
>
> Code-wise this looks alright to me, albeit I did not look at it in-depth,
> but what I'd be interested is documenting some more thoughts about how
> the size of the cache was chosen; even if it was mostly random then stating
> so can help a lot when rethinking this in the future, as then one doesn't
> have to guess if there was some more reasoning behind that.
>
> Also some basic benchmarks might be great, even if from some random grown
> setup, as long as one describes it, like the overall pool data usage,
> deduplication factor, amount of backup groups, amount of snapshots and
> their rough age (distribution) and basic system characteristics like the
> cpu and basic parameters of the underlying storage, like filesystem type
> and (block) device type that backs it, as with that one can classify the
> change somewhat good enough.
>
>
>> Fixes: https://bugzilla.proxmox.com/show_bug.cgi?id=5331
>> Signed-off-by: Christian Ebner <c.ebner@proxmox.com>
>> ---
>> changes since version 3:
>> - no changes
>>
>> pbs-datastore/src/datastore.rs | 26 ++++++++++++++++++++++++--
>> 1 file changed, 24 insertions(+), 2 deletions(-)
>>
>> diff --git a/pbs-datastore/src/datastore.rs b/pbs-datastore/src/datastore.rs
>> index ea7e5e9f3..4445944c0 100644
>> --- a/pbs-datastore/src/datastore.rs
>> +++ b/pbs-datastore/src/datastore.rs
>
> ...
>
>> @@ -1128,6 +1136,8 @@ impl DataStore {
>> let mut unprocessed_index_list = self.list_index_files()?;
>> let index_count = unprocessed_index_list.len();
>>
>> + // Allow up to 32 MiB, as only storing the 32 digest as key
>
> Above comment is IMO a bit hard to parse and does not really provide any
> reasoning about the chosen size FWICT.
>
>> + let mut recently_touched_chunks = LruCache::new(1024 * 1024);
>
> It's quite a descriptive and good name, but something slightly shorter
> like `chunk_lru_cache` would be IMO fine here too, but really no hard
> feelings.
Okay, will adapt this and the other suggestions and use the testlab
datastore to generate the benchmarks as requested. Thanks!
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