Re: os io unit , db block size relation

Hi Fabrizio,

Thank you very much for your help.

I have made a small test:

[oracle_at_tanidw1 tolga]$ time strace -T dd if=/oracle/ett/data/current/tanduyurus
ure.txt of=/dev/null bs=10485760 count=5

Here i try to read 5 10MB. blocks from a file which is stored in ext3 file system.The file system is also part of raid10 array which is EMC . We use 960kb. of stripe size and have one io channel.

Before running this command , i execute iostat -x /dev/sdm1 20 on another terminal .

Here is the strace output :

read(0, "DUYURU_KOD|UYE_ISYERI_KOD|SUBE_K"..., 10485760) = 10485760 <0.594360>
write(1, "DUYURU_KOD|UYE_ISYERI_KOD|SUBE_K"..., 10485760) = 10485760 <0.000008>
read(0, "42\n3133|2|33|485|20040914000000|"..., 10485760) = 10485760 <0.409270>
write(1, "42\n3133|2|33|485|20040914000000|"..., 10485760) = 10485760 <0.000012>
read(0, "1508|20040914000000|094029|8|1|1"..., 10485760) = 10485760 <0.462430>
write(1, "1508|20040914000000|094029|8|1|1"..., 10485760) = 10485760 <0.000012>
read(0, "124457\n896|497|1|3352|2004091400"..., 10485760) = 10485760 <0.437568>
write(1, "124457\n896|497|1|3352|2004091400"..., 10485760) = 10485760 <0.000013>
read(0, "1|2|99|1054|20040914000000|17451"..., 10485760) = 10485760 <0.317013>
write(1, "1|2|99|1054|20040914000000|17451"..., 10485760) = 10485760 <0.000008>
munmap(0x2000000000308000, 10534912) = 0 <0.000611>

real    0m3.075s
user    0m0.006s
sys     0m0.088s

And , iostat values :

avg-cpu: %user %nice %sys %idle

         253.93 0.00 12.25 757.83

Device: rrqm/s wrqm/s r/s w/s rsec/s wsec/s avgrq-sz avgqu-sz await svctm %util
sdm1 590.70 0.05 100.05 0.40 5526.00 3.60 55.05 6.81  67.80 1.51 15.53

It is clear that the dd command reads 50MB. I want to see this in iostat command :

iostat says 5526.00 sectors are read. So this is 2763 KB. (5526*521bytes )

I dont know which part i missed? (May be i forgot the simple math !)

And , iostat says , the system makes 100 read calls per second . This is 55 sectors per read . This 100 read call is not what i issued. Since strace output shows read function calls , i consider the os counts the number of this read calls as io but i think it is not correct. What exactly does iostat count as read call?

I will be appreciated if you can help me about the issue.

Kind Regards,
tolga

FM <fabrizio.magni_at_mycontinent.com> wrote in message news:<4146fd2d$1_at_x-privat.org>...
> utkanbir wrote:
>
>
> > statfs call against the filesystems which oracle data resides returns
> > 512 as the optimal block size.
> >
>
> On ocfs you are not buffering so, I believe, the value you read in
> f_bsize is the physical block (512 byte).
> (You are accessing the DMA "layer" directly).
>
> > When i create a directory in one of the ocfs file systems , its size
> > is 32K.
> >
> > sar -d reports some values in blks/s .
> >
> > I want to connect all these info but i cant. Here is my questions:
> >
> > 1. How can i find the os io unit ? How much data does (not only in blocks ,
> > but also in bytes) the os transfer in each read ?
> >
>
> You are stuck to PAGE_SIZE for buffered i/o: 4k for x86 architectures
> (even x86-64), 16k (by default) on ia64.
>
> > 2. Since i use ocfs which use direct io (no os buffering) , can i say
> > that all database ios are 16kb? (Since this is db block size) Or are
> > all ios issued by db server 512 bytes?(Since statfs returns 512 as the
> > optimal block size)Or are all ios issued by db server 32kb. (this is
> > the ocfs block size)
> >
>
> Since it is not buffered you are not using the virtual memory machine
> (or so I read) so every oracle i/o is made in a single operation.
> (Even the read-ahead is not triggered).
>
> > 3. I want to monitor the raid subsytem by comparing the disk io done
> > by dbserver and os. I use v$filestat oracle table to get the number of
> > disk reads in block , than sar -d or iostat but how can i understand
> > what os block exactly mean in sar and iostat outputs? 16kb, 512bytes
> > or 32kb?
> >
> >
>
> sar and iostat show you reads and writes as sector units so 512 bytes
> (kernel from 2.2 on).
Received on Wed Sep 15 2004 - 02:32:50 CDT