Theodore Y. Ts'o wrote on 2020/12/9 12:34:
> 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@xxxxxxxxxxx>
>>>>
>>>> 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?
Maybe I missed something. If i% meta_bg_size is used instead, if
flex_size <64, then we will miss some flex_bg. There seems to be
a contradiction here. In the scenario where only flex_bg is
enabled, it may not be appropriate to use meta_bg_size. In the
scenario where only meta_bg is enabled, it may not be appropriate
to use flex_size.
As you said before, it maybe better to remove
if ((i <5) || ((i% flex_size) == 0))
and do it for all groups.
In this way we won't miss some flex_bg, meta_bg, and sparse_bg.
I tested it on an 80T disk and found that the performance loss
was small:
unpatched kernel:
ext4_setup_system_zone() takes 524ms,
mount-3137 [006] .... 89.548026: ext4_setup_system_zone: (ext4_setup_system_zone+0x0/0x3f0)
mount-3137 [006] d... 90.072895: ext4_setup_system_zone_1: (ext4_fill_super+0x2057/0x39b0 <- ext4_setup_system_zone)
patched kernel:
ext4_setup_system_zone() takes 552ms,
mount-4425 [006] .... 402.555793: ext4_setup_system_zone: (ext4_setup_system_zone+0x0/0x3d0)
mount-4425 [006] d... 403.107307: ext4_setup_system_zone_1: (ext4_fill_super+0x2057/0x39b0 <- ext4_setup_system_zone)
>
> - Ted
>
