[Gluster-devel] Disperse volume : Sequential Writes
Xavier Hernandez
xhernandez at datalab.es
Tue Jul 4 08:09:29 UTC 2017
Hi Pranith,
On 03/07/17 05:35, Pranith Kumar Karampuri wrote:
> Ashish, Xavi,
> I think it is better to implement this change as a separate
> read-after-write caching xlator which we can load between EC and client
> xlator. That way EC will not get a lot more functionality than necessary
> and may be this xlator can be used somewhere else in the stack if possible.
while this seems a good way to separate functionalities, it has a big
problem. If we add a caching xlator between ec and *all* of its
subvolumes, it will only be able to cache encoded data. So, when ec
needs the "cached" data, it will need to issue a request to each of its
subvolumes and compute the decoded data before being able to use it, so
we don't avoid the decoding overhead.
Also, if we want to make the xlator generic, it will probably cache a
lot more data than ec really needs. Increasing memory footprint
considerably for no real use.
Additionally, this new xlator will need to guarantee that the cached
data is current, so it will need its own locking logic (that would be
another copy&paste of the existing logic in one of the current xlators)
which is slow and difficult to maintain, or it will need to intercept
and reuse locking calls from parent xlators, which can be quite complex
since we have multiple xlator levels where locks can be taken, not only ec.
This is a relatively simple change to make inside ec, but a very complex
change (IMO) if we want to do it as a stand-alone xlator and be generic
enough to be reused and work safely in other places of the stack.
If we want to separate functionalities I think we should create a new
concept of xlator which is transversal to the "traditional" xlator stack.
Current xlators are linear in the sense that each one operates only at
one place (it can be moved by reconfiguration, but once instantiated, it
always work at the same place) and passes data to the next one.
A transversal xlator (or maybe a service xlator would be better) would
be one not bound to any place of the stack, but could be used by all
other xlators to implement some service, like caching, multithreading,
locking, ... these are features that many xlators need but cannot use
easily (nor efficiently) if they are implicitly implemented in some
specific place of the stack outside its control.
The transaction framework we already talked, could be though as one of
these service xlators. Multithreading could also benefit of this
approach because xlators would have more control about what things can
be processed by a background thread and which ones not. Probably there
are other features that could benefit from this approach.
In the case of brick multiplexing, if some xlators are removed from each
stack and loaded as global services, most probably the memory footprint
will be lower and the resource usage more optimized.
Just an idea...
Xavi
>
> On Fri, Jun 16, 2017 at 4:19 PM, Ashish Pandey <aspandey at redhat.com
> <mailto:aspandey at redhat.com>> wrote:
>
>
> I think it should be done as we have agreement on basic design.
>
> ------------------------------------------------------------------------
> *From: *"Pranith Kumar Karampuri" <pkarampu at redhat.com
> <mailto:pkarampu at redhat.com>>
> *To: *"Xavier Hernandez" <xhernandez at datalab.es
> <mailto:xhernandez at datalab.es>>
> *Cc: *"Ashish Pandey" <aspandey at redhat.com
> <mailto:aspandey at redhat.com>>, "Gluster Devel"
> <gluster-devel at gluster.org <mailto:gluster-devel at gluster.org>>
> *Sent: *Friday, June 16, 2017 3:50:09 PM
> *Subject: *Re: [Gluster-devel] Disperse volume : Sequential Writes
>
>
>
>
> On Fri, Jun 16, 2017 at 3:12 PM, Xavier Hernandez
> <xhernandez at datalab.es <mailto:xhernandez at datalab.es>> wrote:
>
> On 16/06/17 10:51, Pranith Kumar Karampuri wrote:
>
>
>
> On Fri, Jun 16, 2017 at 12:02 PM, Xavier Hernandez
> <xhernandez at datalab.es <mailto:xhernandez at datalab.es>
> <mailto:xhernandez at datalab.es
> <mailto:xhernandez at datalab.es>>> wrote:
>
> On 15/06/17 11:50, Pranith Kumar Karampuri wrote:
>
>
>
> On Thu, Jun 15, 2017 at 11:51 AM, Ashish Pandey
> <aspandey at redhat.com <mailto:aspandey at redhat.com>
> <mailto:aspandey at redhat.com <mailto:aspandey at redhat.com>>
> <mailto:aspandey at redhat.com
> <mailto:aspandey at redhat.com> <mailto:aspandey at redhat.com
> <mailto:aspandey at redhat.com>>>> wrote:
>
> Hi All,
>
> We have been facing some issues in disperse (EC)
> volume.
> We know that currently EC is not good for random
> IO as it
> requires
> READ-MODIFY-WRITE fop
> cycle if an offset and offset+length falls in
> the middle of
> strip size.
>
> Unfortunately, it could also happen with
> sequential writes.
> Consider an EC volume with configuration 4+2.
> The stripe
> size for
> this would be 512 * 4 = 2048. That is, 2048
> bytes of user data
> stored in one stripe.
> Let's say 2048 + 512 = 2560 bytes are already
> written on this
> volume. 512 Bytes would be in second stripe.
> Now, if there are sequential writes with offset
> 2560 and of
> size 1
> Byte, we have to read the whole stripe, encode
> it with 1
> Byte and
> then again have to write it back.
> Next, write with offset 2561 and size of 1 Byte
> will again
> READ-MODIFY-WRITE the whole stripe. This is
> causing bad
> performance.
>
> There are some tools and scenario's where such
> kind of load is
> coming and users are not aware of that.
> Example: fio and zip
>
> Solution:
> One possible solution to deal with this issue is
> to keep
> last stripe
> in memory.
> This way, we need not to read it again and we
> can save READ fop
> going over the network.
> Considering the above example, we have to keep
> last 2048 bytes
> (maximum) in memory per file. This should not
> be a big
> deal as we already keep some data like xattr's
> and size info in
> memory and based on that we take decisions.
>
> Please provide your thoughts on this and also if
> you have
> any other
> solution.
>
>
> Just adding more details.
> The stripe will be in memory only when lock on the
> inode is active.
>
>
> I think that's ok.
>
> One
> thing we are yet to decide on is: do we want to read
> the stripe
> everytime we get the lock or just after an extending
> write is
> performed.
> I am thinking keeping the stripe in memory just after an
> extending write
> is better as it doesn't involve extra network operation.
>
>
> I wouldn't read the last stripe unconditionally every
> time we lock
> the inode. There's no benefit at all on random writes
> (in fact it's
> worse) and a sequential write will issue the read anyway
> when
> needed. The only difference is a small delay for the
> first operation
> after a lock.
>
>
> Yes, perfect.
>
>
>
> What I would do is to keep the last stripe of every
> write (we can
> consider to do it per fd), even if it's not the last
> stripe of the
> file (to also optimize sequential rewrites).
>
>
> Ah! good point. But if we remember it per fd, one fd's
> cached data can
> be over-written by another fd on the disk so we need to also
> do cache
> invalidation.
>
>
> We only cache data if we have the inodelk, so all related fd's
> must be from the same client, and we'll control all its writes
> so cache invalidation in this case is pretty easy.
>
> There exists the possibility to have two fd's from the same
> client writing to the same region. To control this we would need
> some range checking in the writes, but all this is local, so
> it's easy to control it.
>
> Anyway, this is probably not a common case, so we could start by
> caching only the last stripe of the last write, ignoring the fd.
>
> May be implementation should consider this possibility.
> Yet to think about how to do this. But it is a good point.
> We should
> consider this.
>
>
> Maybe we could keep a list of cached stripes sorted by offset in
> the inode (if the maximum number of entries is small, we could
> keep the list not sorted). Each fd should store the offset of
> the last write. Cached stripes should have a ref counter just to
> account for the case that two fd's point to the same offset.
>
> When a new write arrives, we check the offset stored in the fd
> and see if it corresponds to a sequential write. If so, we look
> at the inode list to find the cached stripe, otherwise we can
> release the cached stripe.
>
> We can limit the number of cached entries and release the least
> recently used when we reach some maximum.
>
>
> Yeah, this works :-).
> Ashish,
> Can all of this be implemented by 3.12?
>
>
>
>
>
>
> One thing I've observed is that a 'dd' with block size
> of 1MB gets
> split into multiple 128KB blocks that are sent in
> parallel and not
> necessarily processed in the sequential order. This
> means that big
> block sizes won't benefit much from this optimization
> since they
> will be seen as partially non-sequential writes. Anyway
> the change
> won't hurt.
>
>
> In this case as per the solution we won't cache anything
> right? Because
> we didn't request anything from the disk. We will only keep
> the data in
> cache if it is not aligned write which is at the current
> EOF. At least
> that is what I had in mind.
>
>
> Suppose we are writing multiple 1MB blocks at offset 1. If each
> write is split into 8 blocks of 128KB, all writes will be not
> aligned, and can be received in any order. Suppose that the
> first write happens to be at offset 128K + 1. We don't have
> anything cached, so we read the needed stripes and cache the
> last one. Now the next write is at offset 1. In this case we
> won't get any benefit from the previous write, since the stripe
> we need is not cached. However the write from the user point of
> view is sequential.
>
> It won't hurt but it won't take all benefits from the new
> caching mechanism.
>
> As a mitigating factor, we could consider to extend the previous
> solution I've explained to allow caching multiple stripes per
> fd. A small number like 8 would be enough.
>
> Xavi
>
>
>
>
> Xavi
>
>
>
>
>
> ---
> Ashish
>
>
>
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>
> --
> Pranith
>
>
>
>
>
> --
> Pranith
>
>
>
>
>
> --
> Pranith
>
>
>
>
> --
> Pranith
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