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Date:   Tue, 25 May 2021 16:48:01 -0600
From:   Andreas Dilger <adilger@...ger.ca>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Josh Triplett <josh@...htriplett.org>,
        David Howells <dhowells@...hat.com>,
        Theodore Ts'o <tytso@....edu>,
        "Darrick J. Wong" <djwong@...nel.org>, Chris Mason <clm@...com>,
        Ext4 Developers List <linux-ext4@...r.kernel.org>,
        xfs <linux-xfs@...r.kernel.org>,
        linux-btrfs <linux-btrfs@...r.kernel.org>,
        linux-cachefs@...hat.com,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        NeilBrown <neilb@...e.com>
Subject: Re: How capacious and well-indexed are ext4, xfs and btrfs
 directories?

On May 25, 2021, at 3:26 PM, Matthew Wilcox <willy@...radead.org> wrote:
> 
> On Tue, May 25, 2021 at 03:13:52PM -0600, Andreas Dilger wrote:
>> Definitely "-o discard" is known to have a measurable performance impact,
>> simply because it ends up sending a lot more requests to the block device,
>> and those requests can be slow/block the queue, depending on underlying
>> storage behavior.
>> 
>> There was a patch pushed recently that targets "-o discard" performance:
>> https://patchwork.ozlabs.org/project/linux-ext4/list/?series=244091
>> that needs a bit more work, but may be worthwhile to test if it improves
>> your workload, and help put some weight behind landing it?
> 
> This all seems very complicated.  I have chosen with my current laptop
> to "short stroke" the drive.  That is, I discarded the entire bdev,
> then partitioned it roughly in half.  The second half has never seen
> any writes.  This effectively achieves the purpose of TRIM/discard;
> there are a lot of unused LBAs, so the underlying flash translation layer
> always has plenty of spare space when it needs to empty an erase block.
> 
> Since the steady state of hard drives is full, I have to type 'make clean'
> in my build trees more often than otherwise and remember to delete iso
> images after i've had them lying around for a year, but I'd rather clean
> up a little more often than get these weird performance glitches.
> 
> And if I really do need half a terabyte of space temporarily, I can
> always choose to use the fallow range for a while, then discard it again.

Sure, that's one solution for a 1TB laptop, but not large filesystems
that may be hundreds of TB per device.  I don't think the owners of
Perlmutter (https://www.nersc.gov/systems/perlmutter/) could be convinced
to avoid using 17PB of their flash to avoid the need for TRIM to work. :-)

Cheers, Andreas






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