great follow-up chris!
On Tue, Sep 18, 2018 at 9:24 AM Chris Taylor <
christopherdtaylor1994@xxxxxxxxx> 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> 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>
Sent: 31 August 2018 16:04:20
To: Jonathan Lewis
Cc: 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>> 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>
<oracle-l-bounce@xxxxxxxxxxxxx<mailto:oracle-l-bounce@xxxxxxxxxxxxx>> on
behalf of Chris Taylor <christopherdtaylor1994@xxxxxxxxx<mailto:
christopherdtaylor1994@xxxxxxxxx>>
Sent: 31 August 2018 14:23:46
To: 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>>> 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>>> 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
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