On 26.05.19 17:01, Alberto Garcia wrote: > On Fri 24 May 2019 03:56:21 PM CEST, Max Reitz <mre...@redhat.com> wrote: >>> +-----------+-------+------+-------+------+------+ >>> | file | before | after | gain | >>> +-----------+-------+------+-------+------+------+ >>> | ssd | 61.153 | 36.313 | 41% | >>> | hdd | 112.676 | 122.056 | -8% | >>> +-----------+--------------+--------------+------+ >> >> I’ve done a few more tests, and I’ve seen more slowdown on an HDD. >> (Like 30 % when doing 64 kB requests that are not aligned to >> clusters.) On the other hand, the SSD gain is generally in the same >> ballpark (38 % when issuing the same kind of requests.) > [...] >> [1] Hm. We can probably investigate whether the file is stored on a >> rotational medium or not. Is there a fundamental reason why this >> patch seems to degrade performance on an HDD but improves it on an >> SSD? If so, we can probably make a choice based on that. > > This is when writing to an unallocated cluster with no existing data on > the backing image, right? Then it's probably because you need 2 > operations (write zeros + write data) instead of just one.
Hm, yes. I didn’t test writing tail and head separately, which should be even worse. Max
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