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Posted to user@zookeeper.apache.org by Dejan Markic <De...@mobik-ics.com> on 2015/04/16 10:17:20 UTC

Transaction logs and snapshots

Hello all!

We are running 3 ZK servers in ensemble, and ZK is processing a lot of commands per seconds. There are probably around 300 nodes created/checked/set/get per second.
Since we have only information about live sessions we handle in ZK, we don't need any data persistency - eg: we can stop all nodes, clean all transaction logs/snapshots, and start them up again, without any issues.
Since we have a lot of requests/changes, we have moved dataDir onto ramdisk, so we have no problems with disk IOPS, etc.
Is there a way, to minimze the usage of snapshots/logs so ramdisk would not get filled up? It happens that transaction logs/snapshots grow so large, that we run out of space on ramdisk.
We issue >/usr/share/zookeeper/bin/zkCleanup.sh -n 3< every 2 minutes, so this should cleanup the dataDir quite often. Why is >count number of snapshots/logs to keep< limited to 3 and not below?
I assume, in my setup, I don't even need snapshots/logs to be stored after they are not actively needed? 
So my basic questions are:  
- can I somehow get rid of snapshot/logs sooner, more often ... ?
- when is snapshot created? Can it be created sooner, so it would be smaller? 
- Is it possible to get rid of snapshot/logs all together?

Thank you for all your inputs and kind regards,
Dejan Markic

Re: Transaction logs and snapshots

Posted by Flavio Junqueira <fp...@yahoo.com.INVALID>.

Disabling forceSync will only make the writes to the txn log asynchronous, but the same volume of data will be written. I still think you could try to reduce the number of snapshots generated by increasing snapCount.

-Flavio

> On 17 Apr 2015, at 20:51, Michi Mutsuzaki <mu...@gmail.com> wrote:
> 
> Hi Dejan,
> 
> I had a similar usecase: no durability requirement / virtualized (esx)
> environment. We saw intermittent session expiry, so we ended up
> setting forceSync to false. It's been working well since then.
> 
> http://zookeeper.apache.org/doc/trunk/zookeeperAdmin.html#Unsafe+Options
> 
> On Thu, Apr 16, 2015 at 10:08 PM, Dejan Markic
> <De...@mobik-ics.com> wrote:
>> Hello Flavio!
>> 
>> When we were testing ZooKeeper, we saw high IOPS - and since we don't care about data durability, we simply moved it to ramdisk. All ZK's are running on virtual machines (some HyperV, some vmWare). So yes, in the end, any high IOPS can be problematic.
>> So I guess my only solution at the moment is, to increase the ramdisk to accommodate the logs/snapshots.
>> I've just had another idea ... ZK uses only the log file while running right? That's where all IOPS are happening? Is there a way, to put active log on ramdisk, snapshots and old logs to another directory?
>> Don't know why I put snapshots on ramdisk ... if I understand correctly, snapshots are simply written when needed, right? I know I can put snapshots to another directory (eg to disk directly) and it will not cause constant IOPS, right?
>> 
>> Thank you and kind regards,
>> Dejan Markic
>> ________________________________________
>> From: Flavio Junqueira [fpjunqueira@yahoo.com]
>> Sent: Thursday, April 16, 2015 11:26 PM
>> To: Dejan Markic
>> Cc: user@zookeeper.apache.org
>> Subject: Re: Transaction logs and snapshots
>> 
>> Distributed locks is indeed part of our bread and butter. Why don't you want to write to disk? Your workload does't seem to be heavy. Does the IO traffic compete with some other traffic you have?
>> 
>> -Flavio
>> 
>>> On 16 Apr 2015, at 22:15, Dejan Markic <De...@mobik-ics.com> wrote:
>>> 
>>> Hello Flavio!
>>> 
>>> Yes, indeed, ZK might not be the best option - but I could not find any better. What we need is a rather fast, distributed locking "system". ZK was at the moment the best option, and after testing it seemed to be the thing we are looking for. Other than snapshots/transaction logs, we have no problems. It easily handles our current load. It has C library, which makes it fairly easy to port it to other software.
>>> What we need (but I cannot find any) is distributed in-memory distributed locking system where we can store some small information.
>>> For instance, we use ZK's nodes as /SESSION_ID ... we lock it here, and then we use eg /SESSION_ID/my_var to store something. After session is gone, we remove this node and all information about it.
>>> 
>>> If you have any idea about what kind of software we should try, please let me know. You've helped me enough already!
>>> 
>>> Thank you and kind regards,
>>> Dejan Markic
>>> ________________________________________
>>> From: Flavio Junqueira [fpjunqueira@yahoo.com]
>>> Sent: Thursday, April 16, 2015 10:29 PM
>>> To: Dejan Markic
>>> Cc: user@zookeeper.apache.org
>>> Subject: Re: Transaction logs and snapshots
>>> 
>>> Another think you could do is to make snapCount very large so that snapshots are created infrequently. But, let me step back and ask you why you think ZK is a good fit for your project. It isn't clear to me that your case is a good one for ZK.
>>> 
>>> -Flavio
>>> 
>>> 
>>>> On 16 Apr 2015, at 11:01, Dejan Markic <De...@mobik-ics.com> wrote:
>>>> 
>>>> Hello!
>>>> 
>>>> Log seems to be always 67.108.880 bytes.
>>>> Snapshots are currently between 30-40MB. Snapshot is created almost every minute.
>>>> Yes, data durability is not important at all. Once the session ends (it may last between 0 and few minutes, average around 1-2 minutes maybe), I don't need it anymore. I regulary remove  nodes that are not changed for more than 10 minutes.
>>>> I even recieve updates for sessions, so even if ZK looses data, I would get it back after few minutes.
>>>> 
>>>> Thanks!
>>>> 
>>>> Kind regards,
>>>> Dejan
>>>> 
>>>> 
>>>> -----Original Message-----
>>>> From: Flavio Junqueira [mailto:fpjunqueira@yahoo.com.INVALID]
>>>> Sent: Thursday, April 16, 2015 11:49 AM
>>>> To: user@zookeeper.apache.org
>>>> Subject: Re: Transaction logs and snapshots
>>>> 
>>>> Hi Dejan,
>>>> For a typical ZK application, granularity of hours is more than enough, since it is supposed to be an infrequent background task. In your case, it sounds like durability isn't an important property because if it is you shouldn't be getting rid of disk data this fast. I'm also wondering about the amount of data you're generating. What's the size of your snapshots and txn logs?
>>>> -Flavio
>>>> 
>>>> 
>>>>   On Thursday, April 16, 2015 10:26 AM, Dejan Markic <De...@mobik-ics.com> wrote:
>>>> 
>>>> 
>>>> 
>>>> Hello Flavio!
>>>> 
>>>> Would that mean, that zkCleanup.sh would not be needed?
>>>> PurgeInterval is minimum 1 hour? Why is it so high?
>>>> 
>>>> Thanks!
>>>> 
>>>> Kind regards,
>>>> Dejan Markic
>>>> 
>>>> 
>>>> -----Original Message-----
>>>> From: Flavio Junqueira [mailto:fpjunqueira@yahoo.com.INVALID]
>>>> Sent: Thursday, April 16, 2015 11:15 AM
>>>> To: user@zookeeper.apache.org
>>>> Subject: Re: Transaction logs and snapshots
>>>> 
>>>> Hi Dejan,
>>>> Check if the autopurge feature solves your problem:
>>>> http://zookeeper.apache.org/doc/r3.4.6/zookeeperAdmin.html#sc_advancedConfiguration
>>>> 
>>>> -Flavio
>>>> 
>>>> 
>>>>   On Thursday, April 16, 2015 9:17 AM, Dejan Markic <De...@mobik-ics.com> wrote:
>>>> 
>>>> 
>>>> 
>>>> Hello all!
>>>> 
>>>> We are running 3 ZK servers in ensemble, and ZK is processing a lot of commands per seconds. There are probably around 300 nodes created/checked/set/get per second.
>>>> Since we have only information about live sessions we handle in ZK, we don't need any data persistency - eg: we can stop all nodes, clean all transaction logs/snapshots, and start them up again, without any issues.
>>>> Since we have a lot of requests/changes, we have moved dataDir onto ramdisk, so we have no problems with disk IOPS, etc.
>>>> Is there a way, to minimze the usage of snapshots/logs so ramdisk would not get filled up? It happens that transaction logs/snapshots grow so large, that we run out of space on ramdisk.
>>>> We issue >/usr/share/zookeeper/bin/zkCleanup.sh -n 3< every 2 minutes, so this should cleanup the dataDir quite often. Why is >count number of snapshots/logs to keep< limited to 3 and not below?
>>>> I assume, in my setup, I don't even need snapshots/logs to be stored after they are not actively needed?
>>>> So my basic questions are:
>>>> - can I somehow get rid of snapshot/logs sooner, more often ... ?
>>>> - when is snapshot created? Can it be created sooner, so it would be smaller?
>>>> - Is it possible to get rid of snapshot/logs all together?
>>>> 
>>>> Thank you for all your inputs and kind regards, Dejan Markic
>>>> 
>>>> 
>>>> 
>>>> 
>>>> 
>>>> 
>>>> 
>>> 
>>