[Gluster-users] Small files

[Liam Slusser]

Matan -

We replicate to two nodes.  But since a zfs send | zfs recv communicates
one-way, I'd think you could do as many as you want.  It just might take a
little bit longer - although you should be able to run multiple at a time
as long as you had enough bandwidth over the network.  Ours are connected
via a dedicated 10gigabit network and see around 4-5gbit/sec on a large
commit.  How long the replication job takes depends on how much is changed
between the two snapshots.

Even though the seek time with a SSD is quick, you'll still get far greater
throughput in sequential read/writing vs small random accesses.

You can test it yourself.  Create a directory with 100 64MB files and
another directory with 64,000 100K files.  Now copy it from one place to
another and see for yourself which is faster.  Sequential reading always
wins.  And this is true with both Gluster and HDFS.

In HDFS small files exacerbates the problem because you need to contact the
NameNode to get the block information and then contact the DataNode to get
the block.  Think of it like this.  Reading 1000 64KB files in HDFS means
1000 requests to the NameNode and 1000 requests to the datanodes while
reading 1 64MB file is one trip to the NameNode and one trip the the
Datanode to get the same amount of data.

You can read more about this issue here:
http://blog.cloudera.com/blog/2009/02/the-small-files-problem/

thanks,
liam

On Thu, Jan 29, 2015 at 12:30 PM, Matan Safriel <dev.matan at gmail.com> wrote:

> Hi Liam,
>
> Thanks for the comprehensive reply (!)
> How many nodes do you safely replicate to with ZFS?
> I don't think seek time is much of a concern with SSD by the way, so it
> does seem that glusterfs is much better for the small files scenario than
> HDFS, which as you say is very different in key aspects, and couldn't quite
> follow why rebalancing is slow or slower than in the case of HDFS actually,
> unless you just meant that HDFS works at a large block level and no more.
>
> Perhaps you'd care to comment ;)
>
> Matan
>
> On Thu, Jan 29, 2015 at 9:15 PM, Liam Slusser <lslusser at gmail.com> wrote:
>
>> Matan - I'll do my best to take a shot at answering this...
>>
>> They're completely different technologies.  HDFS is not posix compliant
>> and is not a "mountable" filesystem while Gluster is.
>>
>> In HDFS land, every file, directory and block in HDFS is represented as
>> an object in the namenode’s memory, each of which occupies 150 bytes.  So
>> 10 million files would each up about 3 gigs of memory.  Furthermore was
>> designed for streaming large files - the default blocksize in HDFS is 64MB.
>>
>> Gluster doesn't have a central namenode, so having millions of files
>> doesn't put a tax on it in the same way.  But, again, small files causes
>> lots of small seeks to handle the replication tasks/checks and generally
>> isn't very efficient.  So don't expect blazing performance...  Doing
>> rebalancing and rebuilding of Gluster bricks can be extremely painful since
>> Gluster isn't a block level filesystem - so it will have to read each file
>> one at a time.
>>
>> If you want to use HDFS and don't need a mountable filesystem have a look
>> at HBASE.
>>
>> We tacked the small files problem by using a different technology.  I
>> have an image store of about 120 million+ small-file images, I needed a
>> "mountable" filesystem which was posix compliant and ended up doing a ZFS
>> setup - using the built in replication to create a few identical copies on
>> different servers for both load balancing and reliability.  So we update
>> one server and than have a few read-only copies serving the data.  Changes
>> get replicated, at a block level, every few minutes.
>>
>> thanks,
>> liam
>>
>>
>> On Thu, Jan 29, 2015 at 4:29 AM, Matan Safriel <dev.matan at gmail.com>
>> wrote:
>>
>>> Hi,
>>>>
>>>> Is glusterfs much better than hdfs for the many small files scenario?
>>>>
>>>> Thanks,
>>>> Matan
>>>>
>>>
>>>
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>>
>>
>
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