Re: Linux Plumbers IO & File System Micro-conference

[Bernd Schubert <bernd.schubert@itwm.fraunhofer.de>]

On 07/22/2013 02:47 AM, Dave Chinner wrote:
> On Fri, Jul 19, 2013 at 09:52:00PM +0200, Bernd Schubert wrote:
>> Hello Ric, hi all,
>>
>> On 07/12/2013 07:20 PM, Ric Wheeler wrote:
>>>
>>> If you have topics that you would like to add, wait until the
>>> instructions get posted at the link above. If you are impatient, feel
>>> free to email me directly (but probably best to drop the broad mailing
>>> lists from the reply).
>>
>> sorry, that will be a rather long introduction, the short conclusion
>> is below.
>>
>>
>> Introduction to the meta-cache issue:
>> =====================================
>> For quite a while we are redesigning our FhGFS storage layout to
>> workaround meta-cache issues of underlying file systems. However,
>> there are constraints as data and meta-data are distributed on
>> between several targets/servers. Other distributed file systems,
>> such as Lustre and (I think) cepfs should have the similar issues.
>>
>> So the main issue we have is that streaming reads/writes evict
>> meta-pages from the page-cache. I.e. this results in lots of
>> directory-block reads on creating files. So FhGFS, Lustre an (I
>> believe) cephfs are using hash-directories to store object files.
>> Access to files in these hash-directories is rather random and with
>> increasing number of files, access to hash directory-blocks/pages
>> also gets entirely random. Streaming IO easily evicts these pages,
>> which results in high latencies when users perform file
>> creates/deletes, as corresponding directory blocks have to be
>> re-read from disk again and again.
>
> Sounds like a filesystem specific problem. Different filesystems
> have different ways of caching metadata and respond differently to
> page cache pressure.
>
> For example, we changed XFS to have it's own metdata buffer cache
> reclaim mechanisms driven by a shrinker that uses prioritised cache
> reclaim to ensure we reclaim less important metadata buffers before
> ones that are more frequently hit (e.g. to reclaim tree leaves
> before nodes and roots). This was done because the page cache based
> reclaim of metadata was completely inadequate (i.e. mostly random!)
> and would frequently reclaim the wrong thing and cause performance
> under memory pressure to tank....

Well, especially with XFS I see reads all the time and btrace tells me 
these are meta-reads. So far I didn't find a way to make XFS to cache 
meta data permanenly and so far I didn't track that down any further.
For reference and without full bonnie output, with XFS I got about 800 
to 1000 creates/s.
Somewhat that seems to confirm my idea not to let file systems try to 
handle it themselves, but to introduce a generic way to cache meta data.

>
>>  From my point of view, there should be a small, but configurable,
>> number pages reserved for meta-data only. If streaming IO wouldn't
>> be able evict these pages, our and other file systems meta-cache
>> issues probably would be entire solved at all.
>
> That's effectively what XFS does automatically - it doesn't reserve
> pages, but it holds onto the frequently hit metadata buffers much,
> much harder than any other Linux filesystem....
>
>> Example:
>> ========
>>
>> Just a very basic simple bonnie++ test with 60000 files on ext4 with
>> inlined data to reduce block and bitmap lookups and writes.
>>
>> Entirely cached hash directories (16384), which are populated with
>> about 16 million files, so 1000 files per hash-dir.
>>
>>> Version  1.96       ------Sequential Create------ --------Random Create--------
>>> fslab3              -Create-- --Read--- -Delete-- -Create-- --Read--- -Delete--
>>> files:max:min        /sec %CP  /sec %CP  /sec %CP  /sec %CP  /sec %CP  /sec %CP
>>>            60:32:32  1702  14  2025  12  1332   4  1873  16  2047  13  1266   3
>>> Latency              3874ms    6645ms    8659ms     505ms    7257ms    9627ms
>>> 1.96,1.96,fslab3,1,1374655110,,,,,,,,,,,,,,60,32,32,,,1702,14,2025,12,1332,4,1873,16,2047,13,1266,3,,,,,,,3874ms,6645ms,8659ms,505ms,7257ms,9627ms
>
> Command line parameters, details of storage, the scripts you are
> running, etc please. RAM as well, as 16 million files are going to
> require at least 20GB RAM to fully cache...

16 million files are only laying around in the hash directories and are 
not touched at all when new files are created. So I don't know where you 
take 20GB from.
Our file names have a typical size of 21 bytes, so with a classical ext2 
layout that gives 29 bytes, with alignment that makes 32 bytes per 
directory entry. Ignoring '.' and '..' we need 125000 x 4kiB directory 
blocks, so about 500MB + some overhead.

>
> Numbers without context or with "handwavy context" are meaningless
> for the purpose of analysis and understanding.

I just wanted to show here, that creating new files introduces reads 
when meta-data have been evicted from the cache and how easily that can 
happen. From my point of view the hardware does not matter much for that 
purpose.
This was with rotating disks as typically used to store huge amounts of 
HPC data. With SSDs the effect would have been smaller, but even SSDs 
are not as fast as in-memory-cache lookups.

As you are asking:
These are pretty old systems from 2006 with 8GB RAM and 10 rotating 
disks in an (md) raid10.
Our customer systems usually have >=64GiB RAM and often _less_ than 16 
million files per server. But still meta-reads impact latency and 
streaming performance.

The bonnie++ output above basically told the parameters, but here the 
full command for reference:
bonnie++ -s 0 -u 65535 -g 65535 -n60:32:32:1 -d /mnt/fhgfs/

Please not that bonnie++ is not ideally suitable for meta-benchmarks, 
but as I said above, I just wanted to demonstrate cache evictions.


Cheers,
Bernd