Chris,
You have a gift for telling the story. This could make an amazing
newsletter article or a fantastic presentation that you'll enjoy
creating and presenting. The fun part of such an article or
presentation would be explaining the underlying technology, so that the
reader/audience enjoys the punchline as much as we have.
For an article in a newsletter, please consider RMOUG SQL>Update
<http://www.rmoug.org/newsletter/>, the NoCOUG Journal
<http://nocoug.org/newsletter.html>, IOUG Select
<http://select.ioug.org/>, or UKOUG Oracle Scene
<http://www.ukoug.org/membership-new/member-activities/oracle-scene/>?
There might be others I have overlooked, for which I apologize.
Hope this helps!
-Tim
On 9/18/18 08:22, Chris Taylor wrote:
So, I need to own up to something.
My whole approach to this problem was wrong. I had been told that the problem with this table was the CLOB and the number of sessions updating the table at the same time. So, my whole approach was based on that information being TRUE.
I was blinded to the 'real' problem and wasn't actively looking for an alternative answer until I posted this thread. Due to the discussion here, I took a few steps back and actually asked myself "why would this be row locks/blocks?" and WHY don't I see internal locking on the clob if the clob is the problem?
At that point I was in 'question everything' mode and started really looking at the Event that the blocking session was sitting in (instead of just assuming it was a problem with design).
The blocking sessions were ALWAYS sitting in EVENT: SQL*Net MORE DATA FROM CLIENT and WAIT_CLASS: NETWORK and the wait times were reaching upward of 2 minutes before the wait_time_micro restarted counting.
When I saw that (and really looked at it), I was like that makes no sense. Why would a session be getting latency from the PHP server? (Why is it taking so long to send an object from the PHP server to the database?)
Running strace on the sessions on both the db server and the php server showed the sessions just sitting in either read() or write() to the network socket and never completing.
Large file transfers INTO the main db server would stall. (Transfers OUT of the db server were fine) - this affected SQLNet, SCP, ncat etc. Outgoing was fine, Inbound was terrible.
Ultimately we discovered these 2 settings had been DISABLED on this box at some point (as its been around a while):
/proc/sys/net/ipv4/tcp_sack
/proc/sys/net/ipv4/tcp_timestamps
Both of those were "0" whereas all our other db servers those were "1" and weren't experiencing any problem.
After setting both of those to "1" , our sessions now complete and no longer block.
I know some of you may be thinking "How could he miss all the network waits?" - And that's a good question. All I can say is sometimes you get "blinded" by assumptions and aren't really seeing the problem for what it is.
I've known this a long, long time. So, remember kiddos you may have to question 'what you know versus what you think you know' ;)
Also, the very fact of sharing this problem with the you on the list gave me the ability to step back and look at it with a new mindset.
Anyway, I wanted to share that as a testament (and a huge thank you) to the community we have here.
Thanks,
Chris
On Fri, Aug 31, 2018 at 11:48 AM Jonathan Lewis <jonathan@xxxxxxxxxxxxxxxxxx <mailto:jonathan@xxxxxxxxxxxxxxxxxx>> wrote:
If the event is "enq TX: row lock wait" the p1/p2/p3 values will
be about the TX lock in v$lock. p2 = id1, p3 = id2, and p1 will
encode "TX" and the lock mode.
Select p1raw (if its available) or to_char(p1,'XXXXXXXXXXXXXXXX')
and you'll get 00000000054580006 - or possibly a 4 on the end. If
it's a mode 4 lock then that's a clue that it's not really about a
table row and more likely to be about an index/IOT or some
internal anomaly.
If you've got the ASH data then current_obj# may give you the
object_id of the object being accessed, but it's not entirely
reliable.
Regards
Jonathan Lewis
________________________________________
From: Stefan Knecht <knecht.stefan@xxxxxxxxx
<mailto:knecht.stefan@xxxxxxxxx>>
Sent: 31 August 2018 16:04:20
To: Jonathan Lewis
Cc: christopherdtaylor1994@xxxxxxxxx
<mailto:christopherdtaylor1994@xxxxxxxxx>; ORACLE-L
Subject: Re: Looking for ideas on blocked sessions updating
partitioned table with CLOB
You can also look at the p1/p2/p3 values of the sessions that are
blocked by the TX contention - and see which object the contention
is on. If it's the lobindex, that may lead to further clues about
some of the possibilities raised by Jonathan
Stefan
On Fri, Aug 31, 2018 at 8:55 PM, Jonathan Lewis
<jonathan@xxxxxxxxxxxxxxxxxx
<mailto:jonathan@xxxxxxxxxxxxxxxxxx><mailto:jonathan@xxxxxxxxxxxxxxxxxx
<mailto:jonathan@xxxxxxxxxxxxxxxxxx>>> wrote:
There is a major problem with basicfile LOBs (with similar, but
not so drastic symptoms appearing for securefile LOBs), so as a
first strategic step you should probably be planning to find a
painless way to move to securefile LOBs.
I've written an entire system about a problem with basicfile LOBs
when you do inserts and deletes - but the problem is the same if
you do lots of updates because for a LOB an update is a delete
followed by an insert (with the old LOB left in the LOB segment
rather than being copied to the UNDO segment). The way LOBs
handle deletes is that the LOB index is a two-part index of which
the first part indexes in time order the chunks of LOBs that have
been deleted and the second part indexes by lobid the current LOB
chunks.
A problem appears when you try to insert a LOB and there isn't any
free space in the segment, but there is plenty of freeable space.
Your session will free up all the freeable space (deleting LOB
index entries) as it goes. This can take a very long time and
while it's happening any other session that want to insert a LOB
will wait for your session to finish freeing up the space.
Unfortunately I've only see HW enqueues appearing as a result of
this action, I don't think I've seen TX enqueues (and I wouldn't
really expect to see the "enq TX - row lock contention" enqueue).
THe series start at this URL:
http://jonathanlewis.wordpress.com/2017/01/26/basicfile-lobs/
Different possibility - is the LOB declared with multiple
freepools ? This is the default and it means the LOBindex has only
the two parts I describe above. If you have N freepools then the
index consistents of N pairs of parts. This doesn't avoid the
problem I've described above (your session will try to free ALL
the free space from ALL the pools if it needs space), but having a
single freepool may explain your observations. If I insert a very
large LOB PERHAPS you can't insert one until I finish insert
LOBIndex entries into my bit of index, in which case maybe you'd
show a row wait of some sort - though, again I'd expect a
different wait to appear (maybe buffer busy).
Another thought - your LOBs enable in-row storage: how many of
them are short enough to fit in the row (a few, lots, most) and do
they get updated many times in situ, and are many of them likely
to start short and then grow a few times before getting too long
? How long are the rest of the columns in the row ? I'm thinking
about the possibility that your smaller LOBs spend some time
growing in-row and causing row migration before they get long
enough to move into the LOB segment - and row migraion does funny
things with ITLs. Maybe something odd happens as a row that's
migrated moves its LOB to the LOB segment and the row migrates
back to it's original block.
---
In the absence of answers - a plan for the future might be to
range partition by sess_id and hash subpartiiton by sess_id: I'm
assuming that sess_id is a value that's going to increase with
time when I say that. If that is the case then the benefit of
doing this composite partitioning is that on a regular basis you
get a new partition and all the action moves into it and any
garbage that's happened in the earlier partition can be cleared up
(or dropped).
If you stick with basicfiles then look at freepools just in case
it's relevant - securefile lobs automatically have a better
strategy for concurrency but there is a parameter you can set of
increase concurrency. (Can't remember which it is at present).
Since I've mentioned the series - one feature of the mechanism is
that the LOB segment can become much larger than it needs to be,
so if you can sum() the sizes of the lobs that exceed the
in-linesize and it's much smaller than segment size you'll know
that your update mechanism is introducing some sort of problem.
Regards
Jonathan Lewis
Regards
Jonathan Lewis
________________________________________
From: oracle-l-bounce@xxxxxxxxxxxxx
<mailto:oracle-l-bounce@xxxxxxxxxxxxx><mailto:oracle-l-bounce@xxxxxxxxxxxxx
<mailto:oracle-l-bounce@xxxxxxxxxxxxx>>
<oracle-l-bounce@xxxxxxxxxxxxx
<mailto:oracle-l-bounce@xxxxxxxxxxxxx><mailto:oracle-l-bounce@xxxxxxxxxxxxx
<mailto:oracle-l-bounce@xxxxxxxxxxxxx>>> on behalf of Chris Taylor
<christopherdtaylor1994@xxxxxxxxx
<mailto:christopherdtaylor1994@xxxxxxxxx><mailto:christopherdtaylor1994@xxxxxxxxx
<mailto:christopherdtaylor1994@xxxxxxxxx>>>
Sent: 31 August 2018 14:23:46
To: knecht.stefan@xxxxxxxxx
<mailto:knecht.stefan@xxxxxxxxx><mailto:knecht.stefan@xxxxxxxxx
<mailto:knecht.stefan@xxxxxxxxx>>
Cc: ORACLE-L
Subject: Re: Looking for ideas on blocked sessions updating
partitioned table with CLOB
Ah, very good questions.
For the blocking issue:
1. Blocking Type = enq: TX - row lock contention, yet the SESS_IDs
(PK) is different.
2. Basicfile LOBs (I assume these were carried over from an
upgrade from a prev version)
For the PHP object transfer being slow issue:
1. TNS settings:
db server:
sqlnet.ora
---------------------------------
send_buf_size=2097152
recv_buf_size=2097152
default_sdu_size=32768
default_tdu_size=32768
tnsnames.ora
-----------------------------
Nothing on the tnsnames.ora
web/app server:
tnsnames.ora
-------------------------------
Nothing specified here either for SDU etc
2. OS = Red hat Linux 6.8 64-bit , kernel 2.6.32-642.el6.x86_64
3. Block Size = 8192
Thanks,
Chris
On Thu, Aug 30, 2018 at 10:02 PM Stefan Knecht
<knecht.stefan@xxxxxxxxx
<mailto:knecht.stefan@xxxxxxxxx><mailto:knecht.stefan@xxxxxxxxx
<mailto:knecht.stefan@xxxxxxxxx>><mailto:knecht.stefan@xxxxxxxxx
<mailto:knecht.stefan@xxxxxxxxx><mailto:knecht.stefan@xxxxxxxxx
<mailto:knecht.stefan@xxxxxxxxx>>>> wrote:
When you say "blocking" - what event are the blocked sessions
waiting on?
Also, what's your TNS config - particularly SDU sizes between the
mid-tier and the database (TNS connection string and the receiving
listener)?
Are you using securefile or basicfile LOBs?
What OS is the database on and what block size are you using in
the tablespace where the lobs are stored?
On Fri, Aug 31, 2018 at 6:21 AM, Chris Taylor
<christopherdtaylor1994@xxxxxxxxx
<mailto:christopherdtaylor1994@xxxxxxxxx><mailto:christopherdtaylor1994@xxxxxxxxx
<mailto:christopherdtaylor1994@xxxxxxxxx>><mailto:christopherdtaylor1994@xxxxxxxxx
<mailto:christopherdtaylor1994@xxxxxxxxx><mailto:christopherdtaylor1994@xxxxxxxxx
<mailto:christopherdtaylor1994@xxxxxxxxx>>>> wrote:
Env: 12.1.0.2
We have a table that stores session data (base64 encoded) from web
sessions.
The table has 32 HASH partitions and contains a CLOB.
The table is hash partitioned by SESSION_ID which is a 64-char string.
We have this code that executed in PHP into the Oracle database:
UPDATE sessions_table
SET session_data = :session_data // (CLOB)
WHERE session_id = :session_id;
For 95% of the sessions this runs very,very fast.
For 5% of the sessions that LOB being passed from PHP is 15MB and
for some reason PHP is really slow about transmitting those
session OBJECTS over to the database. (Verified through strace etc).
While those sessions are waiting to complete the update to the
CLOB, they end up BLOCKING other sessions that aren't trying to
update the same session id but are in the same partition.
I believe they're blocked on the CLOB on the same partition that
the long running session has open as I've confirmed that the
SESS_ID being updated is different in most cases.
So, here's my question, what performance strategy should I be
investigating for CLOBS. Should I just add more partitions and
spread out the likelihood that a session will end up in the same
partition? Seems logical but I'm not sure how CLOBS play into this.
The CLOBS are stored like this:
Segment Name = SYS_LOB0051640773C00002$$
Index Name = SYS_LOB0051640773C00002$$
Chunk = 8192
PCTVERSION = 10
CACHE = YES
LOGGING = None
Encrypt = None
Compression = None
IN_ROW = Yes
Partition = Yes
Retention = Yes
Looking for any ideas on how to prevent a session from blocking
other non-related sessions that are updating the CLOB.
I'm trying to mitigate the blocking at the db layer while we stand
up a product such as Redis to handle the session caching at the
server level (and remove it from the db).
Chris
--
//
zztat - The Next-Gen Oracle Performance Monitoring and Reaction
Framework!
Visit us at zztat.net
<http://zztat.net><http://zztat.net><http://zztat.net/> |
@zztat_oracle | fb.me/zztat
<http://fb.me/zztat><http://fb.me/zztat><http://fb.me/zztat> |
zztat.net/blog/
<http://zztat.net/blog/><http://zztat.net/blog/><http://zztat.net/blog/>
--
//
zztat - The Next-Gen Oracle Performance Monitoring and Reaction
Framework!
Visit us at zztat.net <http://zztat.net><http://zztat.net/> |
@zztat_oracle | fb.me/zztat
<http://fb.me/zztat><http://fb.me/zztat> | zztat.net/blog/
<http://zztat.net/blog/><http://zztat.net/blog/>
