linux-ext4 - Re: [PATCH RESEND 4/8] ext4: add the gdt block of meta_bg to system

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Date:   Tue, 8 Dec 2020 23:34:15 -0500
From:   "Theodore Y. Ts'o" <tytso@....edu>
To:     brookxu <brookxu.cn@...il.com>
Cc:     adilger.kernel@...ger.ca, linux-ext4@...r.kernel.org
Subject: Re: [PATCH RESEND 4/8] ext4: add the gdt block of meta_bg to
 system_zone

On Fri, Dec 04, 2020 at 09:26:49AM +0800, brookxu wrote:
> 
> Theodore Y. Ts'o wrote on 2020/12/3 23:08:
> > On Sat, Nov 07, 2020 at 11:58:14PM +0800, Chunguang Xu wrote:
> >> From: Chunguang Xu <brookxu@...cent.com>
> >>
> >> In order to avoid poor search efficiency of system_zone, the
> >> system only adds metadata of some sparse group to system_zone.
> >> In the meta_bg scenario, the non-sparse group may contain gdt
> >> blocks. Perhaps we should add these blocks to system_zone to
> >> improve fault tolerance without significantly reducing system
> >> performance.
> 
> Thanks, in the large-market scenario, if we deal with all groups,
> the system_zone will be very large, which may reduce performance.
> I think the previous method is good, but it needs to be changed
> slightly, so that the fault tolerance in the meta_bg scenario
> can be improved without the risk of performance degradation.

OK, I see.   But this is not actually reliable:

> >> +		if ((i < 5) || ((i % flex_size) == 0)) {

This only works if the flex_size is less than or equal to 64 (assuming
a 4k blocksize).  That's because on 64-bit file systems, we can fit 64
block group descripters in a 4k block group descriptor block, so
that's the size of the meta_bg.  The default flex_bg size is 16, but
it's quite possible to create a file system via "mke2fs -t ext4 -G
256".  In that case, the flex_size will be 256, and we would not be
including all of the meta_bg groups.  So i % flex_size needs to be
replaced by "i % meta_bg_size", where meta_bg_size would be
initialized to EXT4_DESC_PER_BLOCK(sb).

Does that make sense?

						- Ted