linux-ext4 - Re: Understanding blktrace; measuring extra writes on ext4 with open(...,O_DSNYC

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Message-ID: <Zq01Nx8SyhoY0/R7@li-bb2b2a4c-3307-11b2-a85c-8fa5c3a69313.ibm.com>
Date: Sat, 3 Aug 2024 01:06:23 +0530
From: Ojaswin Mujoo <ojaswin@...ux.ibm.com>
To: Max Schulze <max.schulze@...ine.de>
Cc: linux-btrace@...r.kernel.org, axboe@...nel.dk, linux-ext4@...r.kernel.org
Subject: Re: Understanding blktrace; measuring extra writes on ext4 with
 open(...,O_DSNYC | O_TRUNC)

On Fri, Aug 02, 2024 at 10:28:47AM +0200, Max Schulze wrote:
> Hello,
> 
> so I have an embedded application with limited flash write cycles. I am writing 2 blocks to disk 5 times a second (ext4, block size 1024).
> The data written is binary and fixed size and to date I open the files with (O_CREAT | O_WRONLY | O_DSYNC | O_TRUNC).
> 
> I set out to measure whether this O_TRUNC leads to an extra write (because I might be able to do without - data is fixed size) and understand the linux tooling around for inspection.
> 
> I wrote a test script, that creates a 2MB ext4 file system on a separate block device, so I can trace with blktrace -d /dev/sdc
> 
> If somebody please could have a look if the interpretation of the traces is correct and what the missing identifiers are.
> 
> 
> Below is the output from blkparse for the moments where I write with open (DSYNC | TRUNC)
> 
> >
> >   8,32   2      152     8.831293895 22900  C RAM 2060 + 2 [0]             <-- previous _C_omplete
> >   8,32  10        2     9.245982628 14449  D   N 0 [kworker/u40:2]        <-- D =issued
> >   8,32   2      153     9.246369033 22900  C   N [0]                      <-- C_omplete
> >   8,32   0      200     9.268019255 15706  A  RM 2062 + 2 <- (8,33) 14    <-- A = remap
> >   8,32   0      201     9.268020922 15706  Q  RM 2062 + 2 [writerdt]      <-- Queued
> >   8,32   0      202     9.268034745 15706  G  RM 2062 + 2 [writerdt]      <-- Get = allocate Req.
> >   8,32   0      203     9.268051167 15706  I  RM 2062 + 2 [writerdt]      <-- I_nserted in Queue
> >   8,32   0      204     9.268107127   161  D  RM 2062 + 2 [kworker/0:1H]  <-- D issued "R"ead
> >   8,32   2      154     9.268704050 22900  C  RM 2062 + 2 [0]
> >   8,32   0      205     9.268881750 15706  A  WS 2160 + 2 <- (8,33) 112
> >   8,32   0      206     9.268882510 15706  Q  WS 2160 + 2 [writerdt]
> >   8,32   0      207     9.268890935 15706  G  WS 2160 + 2 [writerdt]
> >   8,32   0      208     9.268891685 15706  P   N [writerdt]
> >   8,32   0      209     9.268892510 15706  U   N [writerdt] 1
> >   8,32   0      210     9.268895672 15706  I  WS 2160 + 2 [writerdt]
> >   8,32   0      211     9.268913234 15706  D  WS 2160 + 2 [writerdt]      <-- D issued "W"rite "S"ynchronous
> >   8,32   2      155     9.271009859 22900  C  WS 2160 + 2 [0]
> >   8,32   0      212     9.271041534 15706  A WSM 2126 + 2 <- (8,33) 78
> >   8,32   0      213     9.271041910 15706  Q WSM 2126 + 2 [writerdt]
> >   8,32   0      214     9.271051615 15706  G WSM 2126 + 2 [writerdt]
> >   8,32   0      215     9.271053774 15706  I WSM 2126 + 2 [writerdt]
> >   8,32   0      216     9.271075678   161  D WSM 2126 + 2 [kworker/0:1H]  <-- D issued "W"rite "S"ynchronous
> >   8,32   2      156     9.273122709 22900  C WSM 2126 + 2 [0]
> >   8,32   4       67    10.277429577 13420  A  WM 2050 + 2 <- (8,33) 2
> 
> 
> -> What are M and N in the "rwbs" field? I could not find this in the manpage.

Hey Max,

I had the same confusion sometime back and had to dig in the kernel
code. All the rwbs values the latest kernel supports can be found
in this function [1].

So 'M' is added for metadata requests eg when FS is reading some
metadata blocks. 'N' is added when the request is neither
read/write/discard etc. I'm not sure what kind of IO results in N though
so maybe someone else might be able to add to this.

[1]
https://github.com/torvalds/linux/blob/master/kernel/trace/blktrace.c#L1875

(I'll try to go through the trace you provided when i find some time and
update here)

Regards,
ojaswin