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

[Peter Collingbourne via llvm-commits]

As I said that wasn't the limit I was hitting. Here is the program that I
wrote:

#include <stdint.h>
#include <errno.h>
#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>
#include <string.h>

int main() {
  for (uint64_t i = 0;; ++i) {
    char buf[256];
    snprintf(buf, 256, "d/%032lx", i);
    if (i % 65536 == 0)
      puts(buf);
    int fd = open(buf, O_CREAT);
    if (fd == -1) {
      printf("open: %s\n", strerror(errno));
      return 1;
    }
    close(fd);
  }
}

The output ends in:

d/00000000000000000000000000980000
d/00000000000000000000000000990000
d/000000000000000000000000009a0000
d/000000000000000000000000009b0000
d/000000000000000000000000009c0000
d/000000000000000000000000009d0000
d/000000000000000000000000009e0000
d/000000000000000000000000009f0000
open: No space left on device

df:
Filesystem                                              1K-blocks      Used
Available Use% Mounted on
[redacted] 856678580 780069796  33068920  96% [redacted]

df -i:
Filesystem                                                Inodes    IUsed
  IFree IUse% Mounted on
[redacted] 54411264 18824333 35586931   35% [redacted]

Peter


On Tue, Apr 10, 2018 at 1:40 PM, Fangrui Song <maskray at google.com> wrote:

>
> 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-pr
>> oject/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
>>           http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-commits
>>
>>        --
>>        --
>>        Peter
>>
>>    --
>>    宋方睿
>>
>> --
>> --
>> Peter
>>
>
> --
> 宋方睿
>



-- 
-- 
Peter
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