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Date: Thu, 3 Dec 2015 10:58:41 -0700
From: Andreas Dilger <adilger@...ger.ca>
To: lokesh jaliminche <lokesh.jaliminche@...il.com>
Cc: "linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>
Subject: Re: Regarding random grouop search start for allocation of inode.
On Dec 3, 2015, at 01:07, lokesh jaliminche <lokesh.jaliminche@...il.com> wrote:
>
> Thought of giving more clarification on my question
> why group search start is random ? because we can also start search
> for valid groups for inode allocation from the start. As this group
> search is random inode selection might go to end of groups which
> might affect IO performance
Starting the inode search at the beginning of the disk each time
means that inode allocation will be inefficient because it will search
over groups that are mostly or entirely full already.
Allocating the new directory in a semi-random group, one that is
relatively unused, ensures that new
inode and block allocations are relatively efficient afterward.
Cheers, Andreas
> On Thu, Dec 3, 2015 at 1:14 PM, lokesh jaliminche
> <lokesh.jaliminche@...il.com> wrote:
>> hello folks,
>> I am new to ext4 code. I was going through the
>> ext4-source for allocation of inode.
>> There is one thing that I did not understand while selection of groups
>> for inode allocation . I came across this code snippet which is part
>> of find_group_orlov function. question is, why group search start is
>> random ?
>>
>> Code snippet:
>> ==========
>> В·В·В·if (qstr) {
>> »·······»·······»·······hinfo.hash_version = LDISKFS_DX_HASH_HALF_MD4;
>> »·······»·······»·······hinfo.seed = sbi->s_hash_seed;
>> »·······»·······»·······ldiskfsfs_dirhash(qstr->name, qstr->len, &hinfo);
>> »·······»·······»·······grp = hinfo.hash;
>> »·······»·······} else
>> »·······»·······»·······get_random_bytes(&grp, sizeof(grp));
>> »·······»·······parent_group = (unsigned)grp % ngroups;
>> »·······»·······for (i = 0; i < ngroups; i++) {
>> »·······»·······»·······g = (parent_group + i) % ngroups;
>> »·······»·······»·······get_orlov_stats(sb, g, flex_size, &stats);
>> »·······»·······»·······if (!stats.free_inodes)
>> »·······»·······»·······»·······continue;
>> »·······»·······»·······if (stats.used_dirs >= best_ndir)
>> »·······»·······»·······»·······continue;
>> »·······»·······»·······if (stats.free_inodes < avefreei)
>> »·······»·······»·······»·······continue;
>> »·······»·······»·······if (stats.free_blocks < avefreeb)
>> »·······»·······»·······»·······continue;
>> »·······»·······»·······grp = g;
>> »·······»·······»·······ret = 0;
>> »·······»·······»·······best_ndir = stats.used_dirs;
>> »·······»·······}
>>
>> Thanks & Regards,
>> Lokesh
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