[systemd-devel] Antw: [EXT] Re: consider dropping defrag of journals on btrfs

[Ulrich Windl]

>>> Chris Murphy <lists at colorremedies.com> schrieb am 09.02.2021 um 06:13 in
Nachricht
<CAJCQCtRTcCT1kdOrsGHj+idmcVzsSJN+j_jiu9TMXFKWTxQWUA at mail.gmail.com>:
> On Mon, Feb 8, 2021 at 7:56 AM Phillip Susi <phill at thesusis.net> wrote:
>>
>>
>> Chris Murphy writes:
>>
>> >> It sounds like you are arguing that it is better to do the wrong thing
>> >> on all SSDs rather than do the right thing on ones that aren't broken.
>> >
>> > No I'm suggesting there isn't currently a way to isolate
>> > defragmentation to just HDDs.
>>
>> Yes, but it sounded like you were suggesting that we shouldn't even try,
>> not just that it isn't 100% accurate.  Sure, some SSDs will be stupid
>> and report that they are rotational, but most aren't stupid, so it's a
>> good idea to disable the defragmentation on drives that report that they
>> are non rotational.
> 
> So far I've seen, all USB devices report rotational. All USB flash
> drives, and any SSD in an enclosure.
> 
> Maybe some way of estimating rotational based on latency standard
> deviation, and stick that in sysfs, instead of trusting device
> reporting. But in the meantime, the imperfect rule could be do not
> defragment unless it's SCSI/SATA/SAS and it reports it's rotational.

OTOH we had set the rotational attribute to zero for years with out negative
effects.
We did that for SAN storage that actually has spinning discs, but what sense
does the rotational attribute make if the logical disk is distributed over 40
physical disks, managed by a smart controller that does I/O scheduling across
multiple hosts and logical disks?

> 
> ‑‑ 
> Chris Murphy
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