On 9/17/18 11:40 AM, Joshi wrote: > 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? find_group_orlov() randomly selects block groups for root directories, but AFAIK there is no explicit random behavior in the block allocator. It's more of a chaotic system than intentional random selection. :) > End objective of above was to create fragmented file, with repeatable > extent-count/block-numbers. I understand that you want repeatable allocation behavior. I still don't know exactly what behavior you are ultimately trying to test, though. > I'll check xfstests as well. The tests marked as being in the defrag group are: tests/btrfs/005 tests/btrfs/021 tests/btrfs/062 tests/btrfs/067 tests/btrfs/070 tests/btrfs/072 tests/btrfs/074 tests/ext4/020 tests/ext4/301 tests/ext4/302 tests/ext4/303 tests/ext4/304 tests/ext4/307 tests/ext4/308 tests/generic/018 tests/generic/324 -Eric > 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 >>> >> > >
