linux-ext4 - Re: ext4lazyinit reads HDD data on mount since 5.13

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Message-ID: <YRFzWGkBOHcugxGD@mit.edu>
Date:   Mon, 9 Aug 2021 14:26:32 -0400
From:   "Theodore Ts'o" <tytso@....edu>
To:     ValdikSS <iam@...dikss.org.ru>
Cc:     linux-ext4@...r.kernel.org
Subject: Re: ext4lazyinit reads HDD data on mount since 5.13

On Mon, Aug 09, 2021 at 10:43:08AM +0300, ValdikSS wrote:
> On 09.08.2021 04:51, Theodore Ts'o wrote:
> > It's a new feature which optimizes block allocations for very large
> > file systems.  The work being done by ext4lazyinit is to read the
> > block allocation bitmaps so we can cache the buddy bitmaps and how
> > fragmented (or not) various block groups are, which is used to
> > optimize the block allocator.
> 
> Thanks for the info. To revert old behavior, the filesystem should be
> mounted with -o no_prefetch_block_bitmaps
> 
> Is it safe to use this option with new optimizations? Should I expect only
> less optimal filesystem speed and no other issues?

It's not been tested, but it should be safe in terms that it shouldn't
lead to any file system corruption or data loss.  However, it may
result in non-optional block placement that might cause more file or
free-space fragmentation that might otherwise be the case.  (This was
true even before the latest optimizations, but it's more the case with
the new optimizations.)

Can you say something about why you want to disable to block
allocation prefetch?  How is it causing problems for you?

Cheers,

					- Ted