It's not that I need specific values for extents/blocks for a file. It is just about being able to create/prepare a file of given size with same (repeatable) number of extents and block numbers, in a controlled environment. I thought it would happen naturally, if I keep preconditions same (i.e. post format with lazy initialization disabled. I had not disabled lazy-allocation though). But extent count differed, at times. And I wondered whether Ext4 code makes any conscious attempt to randomize the extent/block allocation? End objective of above was to create fragmented file, with repeatable extent-count/block-numbers. I'll check xfstests as well. Thanks, On Mon, Sep 17, 2018 at 9:31 PM Eric Sandeen <sandeen@xxxxxxxxxx> wrote: > > On 9/17/18 10:59 AM, Joshi wrote: > > Sorry if i gave the impression that i needed something in user-mode only. I am fine even if change involves modifying ext4 source. > > Perhaps you can describe what defrag behavior you are trying to test, > and we can offer some suggestions for good ways to construct that test? > > There are already a handful of defrag tests in xfstests, fwiw. > > Thanks, > -Eric > > > On Mon, 17 Sep 2018, 21:14 Eric Sandeen, <esandeen@xxxxxxxxxx <mailto:esandeen@xxxxxxxxxx>> wrote: > > > > On 9/17/18 6:22 AM, Joshi wrote: > > > I've been doing some defrag related tests, and for that I needed to be > > > able to create file with same set of block numbers (i.e. extents), for > > > at least two times. > > > > Userspace cannot control that. > > > > > May I know if there is any randomness in Ext4 allocator, and if there > > > is any, can I disable it for the purpose of getting repeatable > > > block-allocation patterns. > > > > > > Here are experiment details - > > > For a 100K file (created using dd if=/dev/zero of=/mnt/f100k bs=4K > > > count=100, oflag=direct) I got extent info in one run as this - > > > > > > File size of /mnt/file400k is 409600 (100 blocks of 4096 bytes) > > > ext: logical_offset: physical_offset: length: expected: flags: > > > 0: 0.. 15: 34816.. 34831: 16: > > > 1: 16.. 99: 33824.. 33907: 84: 34832: last,eof > > > > > > while in the the second run I got somewhat different runs - > > > File size of /mnt/file400k is 409600 (100 blocks of 4096 bytes) > > > ext: logical_offset: physical_offset: length: expected: flags: > > > 0: 0.. 0: 34816.. 34816: 1: > > > 1: 1.. 15: 34320.. 34334: 15: 34817: > > > 2: 16.. 99: 33824.. 33907: 84: 34335: last,eof > > > > > > Each run beings with a mkfs.ext4 with lazy inode/journal > > > initialization disabled. > > > > Userspace doesn't get to pick physical locations for allocations, you cannot > > write a test which depends on doing so. > > > > -Eric > > > -- Joshi
