Re: "hardware-assisted zeroing"

[Qu Wenruo <quwenruo.btrfs@gmx.com>]

On 2022/1/4 18:50, Eric Levy wrote:
> On Mon, 2022-01-03 at 19:51 +0800, Qu Wenruo wrote:
>
>> The filesystem (normally) doesn't maintain such info, what a
>> filesystem
>> really care is the unused/used space.
>>
>> For fstrim case, the filesystem will issue such discard comand to
>> most
>> (if not all) unused space.
>>
>> And one can call fstrim multiple times to do the same work again and
>> again, the filesystem won't really care.
>> (even the operation can be very time consuming)
>>
>> The special thing in btrfs is, there is a cache to record which
>> blocks
>> have been trimmed. (only in memory, thus after unmount, such cache is
>> lost, and on next mount will need to be rebuilt)
>>
>> This is to reduce the trim workload with recent async-discard
>> optimization.
>
> So in the general case (i.e. no session cache), the trim operation
> scans all the allocation structures, to process all non-allocated
> space?

Yes, and that's almost for all filesystems supporting trim.

All filesystems supporing read-write need to maintain such info anyway.
IIRC for filesystems like ext4, there is a bitmap storing which sector
is used and which is not.

Btrfs has a more complex one (extent tree), not only recording which
range is used, but also which tree is using it.

>
>>> Why is the
>>> command not sent instantly, as soon as the space is freed by the
>>> file
>>> system?
>>
>> If you use discard mount option, then most filesystems will send the
>> discard command to the underlying device when some space is freed.
>>
>> But please keep in mind that, how such discard command gets handled
>> is
>> hardware/storage stack dependent.
>>
>> Some disk firmware may choose to do discard synchronously, which can
>> hugely slow down other operations.
>> (That's why btrfs has async-discard optimization, and also why fstrim
>> is
>> preferred, to avoid unexpected slow down).
>
> Yes, but of course as I have used "instantly", I meant, not necessarily
> synchronously, but simply near in time.
>
> The trim operation is not avoiding bottlenecks, even if it is non-
> blocking, because it operates at the level of the entire file system,
> in a single operation. If Btrfs is able to process discard operations
> asynchronously, then mounting with the discard option seems preferable,
> as it requires no redundant work, adds no serious delay until until the
> calls are made, and depends on no activity (not even automatic
> activity) from the admin.

IIRC this async discard is currently only specific to btrfs, thus it's
not really generic.

Another thing is, just as Zygo said, there is not much benefit from
discarding some frequently used/freed metadata.

But the overhead is always there for discard mount option, thus that's
why we don't recommend discard mount option, even we have async-discard
behavior.

Thanks,
Qu

>
> I fail to see a reason for preferring trim over discard.
>