Date: Wed, 28 Feb 2018 14:16:43 -0500
From: Dianne Skoll <dfs@roaringpenguin.com>
Cc: pgsql-admin@lists.postgresql.org
Subject: Re: Reliable WAL file shipping over unreliable network
Message-ID: <20180228141643.40a8eb8a@hydrogen.roaringpenguin.com>
In-Reply-To: <20180228135220.1cd46347@hydrogen.roaringpenguin.com>
References: <dd116172-3ddf-ee56-57f3-6e18d49abf8e@shopzeus.com>
	<400AC9CD-1E43-422C-A689-70BDBAFF231E@elevated-dev.com>
	<ee3ae4be-ab24-8443-82c1-95bcd4f86a63@shopzeus.com>
	<debd4205-e27a-cd9d-9169-6748e65083a2@shopzeus.com>
	<BA4A1866-95B0-49C6-8CD1-3D1B60A7837E@elevated-dev.com>
	<CF30D15A-BD42-42A7-9AD3-BB69AF9E8047@icloud.com>
	<D5B11CF5-E816-435B-BFB8-1226E23C046A@elevated-dev.com>
	<37C5A42B-4CDD-4F01-8F1E-1B185C1819B9@icloud.com>
	<20180228132620.2c882f84@hydrogen.roaringpenguin.com>
	<33220469-5BB2-4A40-8F56-8F4F392C95B3@icloud.com>
	<20180228135220.1cd46347@hydrogen.roaringpenguin.com>
Organization: Roaring Penguin Software Inc.
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So, I decided to run a test.

It turns out we are both wrong.

My test script is as follows:

#======================================================================
#!/bin/bash

# Remote machine
MACH=dianne.skoll.ca

# Create a 10MB pseudo-random file
echo "`date` Creating 10MB file"
FILE=pseudo-random
dd if=/dev/urandom bs=1k count=10k of=$FILE 2>/dev/null

# Rsync it over at 500kB per second in the background
echo "`date` Starting rsync"
rsync -c --verbose --bwlimit=500k $FILE $MACH:/tmp &
RSYNC_PID=$!

echo "`date` rsync PID=$RSYNC_PID; sleeping for 3 seconds"
# Sleep for 3 seconds
sleep 3

echo "`date` Stopping rsync"
# Stop the rsync
kill -STOP $RSYNC_PID

echo "`date` Changing source file"
# Change the source file
dd if=/dev/urandom bs=1k count=10k of=$FILE 2>/dev/null

echo "`date` Continuing rsync"
# Restart the rsync and bring it into the foreground
kill -CONT $RSYNC_PID
wait

echo "`date` Rsync done; comparing files"

ssh $MACH "cd /tmp && sha1sum $FILE" | sha1sum -c -
#======================================================================

Running it produces this output:

Wed Feb 28 14:14:32 EST 2018 Creating 10MB file
Wed Feb 28 14:14:32 EST 2018 Starting rsync
Wed Feb 28 14:14:32 EST 2018 rsync PID=24456; sleeping for 3 seconds
pseudo-random
Wed Feb 28 14:14:35 EST 2018 Stopping rsync
Wed Feb 28 14:14:35 EST 2018 Changing source file
Wed Feb 28 14:14:35 EST 2018 Continuing rsync

sent 10,488,419 bytes  received 19,505 bytes  488,740.65 bytes/sec
total size is 10,485,760  speedup is 1.00
Wed Feb 28 14:14:53 EST 2018 Rsync done; comparing files
pseudo-random: FAILED
sha1sum: WARNING: 1 computed checksum did NOT match

So even *with* the -c flag, rsync didn't notice the file had been changed
while it was running.  Interesting.

Regards,

Dianne.