linux-ext4 - Re: Regarding ext4 extent allocation strategy

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Message-ID: <CANT5p=pGsU+rgyd-2m+ODOwkxDvdbZKi81DthuuQgzDUXZ9UAQ@mail.gmail.com>
Date:   Wed, 14 Jul 2021 06:07:36 +0530
From:   Shyam Prasad N <nspmangalore@...il.com>
To:     "Theodore Y. Ts'o" <tytso@....edu>
Cc:     David Howells <dhowells@...hat.com>,
        Steve French <smfrench@...il.com>, linux-ext4@...r.kernel.org
Subject: Re: Regarding ext4 extent allocation strategy

On Wed, Jul 14, 2021 at 1:48 AM Theodore Y. Ts'o <tytso@....edu> wrote:
>
> On Tue, Jul 13, 2021 at 06:27:37PM +0530, Shyam Prasad N wrote:
> >
> > Also, is this parameter also respected when a hole is punched in the
> > middle of an allocated data extent? i.e. is there still a possibility
> > that a punched hole does not translate to splitting the data extent,
> > even when extent_max_zeroout_kb is set to 0?
>
> Ext4 doesn't ever try to zero blocks as part of a punch operation.
> It's true a file system is allowed to do it, but I would guess most
> wouldn't, since the presumption is that userspace is actually trying
> to free up disk space, and so you would want to release the disk
> blocks in the punch hole case.
>
> The more interesting one is the FALLOC_FL_ZERO_RANGE_FL operation,
> which *should* work by transitioning the extent to be uninitialized,
> but there might be cases where writing a few zero blocks might be
> faster in some cases.  That should use the same code path which
> resepects the max_zeroout configuration parameter for ext4.
>
> Cheers,
>
>                                         - Ted

Thanks a lot for your replies, Ted. This was useful.

-- 
Regards,
Shyam