[llvm] r329648 - [CachePruning] Fix comment about ext4 per-directory file limit. NFC

[Fangrui Song via llvm-commits]

su
truncate -s 100G 100G
mkfs.ext4 100G
mkdir ext4
mount 100G ext4
cd ext4

mkdir p
cd p
python3 -c 'for i in range(6600000):\n with open(str(i),"w"): pass'


It runs out of inodes with some message like:

OSError: [Errno 28] No space left on device: '6553587'


umount ext4; dumpe2fs 100G # says the inodes are used up
...
Free inodes:              0
...

On 2018-04-10, Peter Collingbourne wrote:
>No, these were empty files. It wasn't an inode limit because I could still
>create files in other directories.
>
>Peter
>
>On Tue, Apr 10, 2018 at 10:35 AM, Fangrui Song <maskray at google.com> wrote:
>
>    On 2018-04-09, Peter Collingbourne wrote:
>
>        Are you sure about that? I'm pretty sure that before writing that
>        comment I
>        wrote a small program that created lots of files (not subdirectories)
>        in a
>        directory until it started getting error messages, which started
>        happening at
>        around 6000000 files.
>
>        Peter
>
>    I guess you created a file of 100GiB. The number of inodes is roughly
>    6553600.
>
>    100*1024*1024*1024 / 16384 = 6553600.0 where 16384 is the default
>    bytes-per-inode (man mke2fs).
>
>    % truncate -s 100G 100G
>    % mkfs.ext4 100G
>    % dumpe2fs 100G
>    .....
>    Inode count:              6553600
>    .....
>
>    Each file consumes one inode and the number of files in that directory
>    is limited by this factor.
>
>        On Mon, Apr 9, 2018 at 5:12 PM, Fangrui Song via llvm-commits <
>        llvm-commits at lists.llvm.org> wrote:
>
>           Author: maskray
>           Date: Mon Apr  9 17:12:28 2018
>           New Revision: 329648
>
>           URL: http://llvm.org/viewvc/llvm-project?rev=329648&view=rev
>           Log:
>           [CachePruning] Fix comment about ext4 per-directory file limit. NFC
>
>           There is a limit on number of subdirectories if dir_nlinks is not
>           enabled (31998), but per-directory number of files is not limited.
>
>           Modified:
>               llvm/trunk/include/llvm/Support/CachePruning.h
>
>           Modified: llvm/trunk/include/llvm/Support/CachePruning.h
>           URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm/
>        Support/
>           CachePruning.h?rev=329648&r1=329647&r2=329648&view=diff
>           ====================================================================
>        =======
>           ===
>           --- llvm/trunk/include/llvm/Support/CachePruning.h (original)
>           +++ llvm/trunk/include/llvm/Support/CachePruning.h Mon Apr  9
>        17:12:28 2018
>           @@ -52,9 +52,8 @@ struct CachePruningPolicy {
>              /// the number of files based pruning.
>              ///
>              /// This defaults to 1000000 because with that many files there
>        are
>           -  /// diminishing returns on the effectiveness of the cache, and
>        some file
>           -  /// systems have a limit on how many files can be contained in a
>           directory
>           -  /// (notably ext4, which is limited to around 6000000 files).
>           +  /// diminishing returns on the effectiveness of the cache, and
>        file
>           +  /// systems have a limit on total number of files.
>              uint64_t MaxSizeFiles = 1000000;
>            };
>
>           _______________________________________________
>           llvm-commits mailing list
>           llvm-commits at lists.llvm.org
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>
>        --
>        -- 
>        Peter
>
>    --
>    宋方睿
>
>--
>-- 
>Peter

-- 
宋方睿