On 2/22/16 8:40 AM, Moreno Andreo wrote:
> Il 18/02/2016 21:33, Jim Nasby ha scritto:
>> Depending on your needs, could could use synchronous replication as
>> part of that setup. You can even do that at a per-transaction level,
>> so maybe you use sync rep most of the time, and just turn it off when
>> inserting or updating BLOBS.
> This sounds good, and when everything is OK we have I/O operation split
> across the two servers; a small delay in synchronizing blobs should not
> be a big deal, even if something bad happens (because of XLOG), right?
It all depends on what you can tolerate. You also don't have to use
synchronous replication; normal streaming replication is async, so if
you can stand to lose some data if one of the servers dies then you can
do that.
>>> Last thing: should blobs (or the whole database directory itself) go in
>>> a different partition, to optimize performance, or in VM environment
>>> this is not a concern anymore?
>>
>> First: IMO concerns about blobs in the database are almost always
>> overblown.
> In many places I've been they say, at last, "BLOBs are slow". So I
> considered this as another point to analyze while designing server
> architecture. If you say "don't mind", then I won't.
It all depends. They're certainly a lot slower than handling a single
int, but in many cases the difference just doesn't matter.
>> 30GB of blobs on modern hardware really isn't a big deal, and there's
>> a *lot* to be said for not having to write the extra code to manage
>> all that by hand.
> What do you mean? Extra code?
If the blob is in the database then you have nothing extra to do. It's
handled just like all your other data.
If it's a file in a file system then you need to:
- Have application code that knows how and where to get at the file
- Have a way to make those files available on all your webservers
- Have completely separate backup and recovery plans for those files
That's a lot of extra work. Sometimes it's necessary, but many times
it's not.
>> When it comes to your disk layout, the first things I'd look at would be:
>>
>> - Move the temporary statistics directory to a RAM disk
>> - Move pg_xlog to it's own partition
> So I need another vDisk, not that big, for pg_xlog?
Yeah, but note that with virtualization that may or may not help.
--
Jim Nasby, Data Architect, Blue Treble Consulting, Austin TX
Experts in Analytics, Data Architecture and PostgreSQL
Data in Trouble? Get it in Treble! http://BlueTreble.com
