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Re: backup manifests - Mailing list pgsql-hackers

From Jeevan Chalke
Subject Re: backup manifests
Date
Msg-id CAM2+6=XD2fyYxnfD5pMy5tOOLT583oezUKupnD5PRm784QYEEg@mail.gmail.com
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In response to Re: backup manifests  (Rushabh Lathia <rushabh.lathia@gmail.com>)
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On Tue, Nov 19, 2019 at 3:30 PM Rushabh Lathia <rushabh.lathia@gmail.com> wrote:


My colleague Suraj did testing and noticed the performance impact
with the checksums.   On further testing, he found that specifically with
sha its more of performance impact.  

Please find below statistics:

no of tableswithout checksumSHA256
checksum		% performnce
overhead
with
SHA-256		md5 checksum% performnce
overhead with md5		CRC checksum% performnce
overhead with
CRC
10 (100 MB
in each table)		real 0m10.957s
user 0m0.367s
sys 0m2.275s		real 0m16.816s
user 0m0.210s
sys 0m2.067s		53%real 0m11.895s
user 0m0.174s
sys 0m1.725s		8%real 0m11.136s
user 0m0.365s
sys 0m2.298s		2%
20 (100 MB
in each table)		real 0m20.610s
user 0m0.484s
sys 0m3.198s		real 0m31.745s
user 0m0.569s
sys 0m4.089s
54%real 0m22.717s
user 0m0.638s
sys 0m4.026s		10%real 0m21.075s
user 0m0.538s
sys 0m3.417s		2%
50 (100 MB
in each table)		real 0m49.143s
user 0m1.646s
sys 0m8.499s		real 1m13.683s
user 0m1.305s
sys 0m10.541s		50%real 0m51.856s
user 0m0.932s
sys 0m7.702s		6%real 0m49.689s
user 0m1.028s
sys 0m6.921s		1%
100 (100 MB
in each table)		real 1m34.308s
user 0m2.265s
sys 0m14.717s		real 2m22.403s
user 0m2.613s
sys 0m20.776s		51%real 1m41.524s
user 0m2.158s
sys 0m15.949s
8%real 1m35.045s
user 0m2.061s
sys 0m16.308s		1%
100 (1 GB
in each table)		real 17m18.336s
user 0m20.222s
sys 3m12.960s		real 24m45.942s
user 0m26.911s
sys 3m33.501s		43%real 17m41.670s
user 0m26.506s
sys 3m18.402s		2%real 17m22.296s
user 0m26.811s
sys 3m56.653s

sometimes, this test
completes within the
same time as without
checksum.		approx. 0.5%


Considering the above results, I modified the earlier Robert's patch and added
"manifest_with_checksums" option to pg_basebackup.  With a new patch.
by default, checksums will be disabled and will be only enabled when
"manifest_with_checksums" option is provided.  Also re-based all patch set.

Review comments on 0004:

1.
I don't think we need o_manifest_with_checksums variable,
manifest_with_checksums can be used instead.

2.
We need to document this new option for pg_basebackup and basebackup.

3.
Also, instead of keeping manifest_with_checksums as a global variable, we
should pass that to the required function. Patch 0002 already modified the
signature of all relevant functions anyways. So just need to add one more bool
variable there.

4.
Why we need a "File" at the start of each entry as we are adding files only?
I wonder if we also need to provide a tablespace name and directory marker so
that we have "Tablespace" and "Dir" at the start.

5.
If I don't provide manifest-with-checksums option then too I see that checksum
is calculated for backup_manifest file itself. Is that intentional or missed?
I think we should omit that too if this option is not provided.

6.
Is it possible to get only a backup manifest from the server? A client like
pg_basebackup can then use that to fetch files reading that.

Thanks
 



Regards,

--
Rushabh Lathia

On Tue, Oct 1, 2019 at 5:43 PM Robert Haas <robertmhaas@gmail.com> wrote:
On Mon, Sep 30, 2019 at 5:31 AM Jeevan Chalke
<jeevan.chalke@enterprisedb.com> wrote:
> Entry for directory is not added in manifest. So it might be difficult
> at client to get to know about the directories. Will it be good to add
> an entry for each directory too? May be like:
> Dir    <dirname> <mtime>

Well, what kind of corruption would this allow us to detect that we
can't detect as things stand? I think the only case is an empty
directory. If it's not empty, we'd have some entries for the files in
that directory, and those files won't be able to exist unless the
directory does. But, how would we end up backing up an empty
directory, anyway?

I don't really *mind* adding directories into the manifest, but I'm
not sure how much it helps.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company




--
Rushabh Lathia


--
Jeevan Chalke
Associate Database Architect & Team Lead, Product Development
EnterpriseDB Corporation
The Enterprise PostgreSQL Company

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