sys.dm_io_virtual_file_stats

Question

sys.dm_io_virtual_file_stats

Leo Leong

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Points: 1615
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January 18, 2009 at 11:16 pm

#133639

Hi,
does anyone know whether columns below in sys.dm_io_virtual_file_stats are accum value since last sql server reboot?
-> num_of_reads, num_of_bytes_read, io_stall_read_ms, num_of_writes, num_of_bytes_written, io_stall_write_ms and io_stall
if yes, is it possible that value of num_of_reads for current date is smaller than previous date? why?
thanks a lot.
leo

Viewing 7 posts - 1 through 6 (of 6 total)

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ChiragNS One Orange Chip Points: 26137 More actions Follow · Answer 1

Although iam not 100% sure it seems like the data is shown from the last server reboot. Our dev servers are shutdown for the weekend and when i checked the data today (monday) it shows very small number of reads/writes.

"Keep Trying"

Leo Leong SSCommitted Points: 1615 More actions · Answer 2

we capture the data from this view every hour.

however, the data looks weird.

at one point, num_of_reads dropped.

Date num_of_reads

1/12/09 11:00 PM 22

1/13/09 11:00 PM 30

1/14/09 11:00 PM 32

1/15/09 11:00 PM 18

1/16/09 11:00 PM 57

1/17/09 11:00 PM 95

ChiragNS One Orange Chip Points: 26137 More actions Follow · Answer 3

If very little activity is going in the db number of reads can go down. If you are reading the data for a particular file did u give the correct fileid as parameter.

One thing iam not sure of is .... when the data is read from file/disk it may be stored in memory. Any subsequent reads of the data might be from the memory and not from file/disk. Maybe someone can throw some light on this.

"Keep Trying"

viswanathanmca Grasshopper Points: 13 More actions · Answer 4

Please run the following and can see I/O statistics of each database file

select db_name(mf.database_id) as databaseName, mf.physical_name,

num_of_reads, num_of_bytes_read, io_stall_read_ms, num_of_writes,

num_of_bytes_written, io_stall_write_ms, io_stall,size_on_disk_bytes

from sys.dm_io_virtual_file_stats(5,null) as divfs

join sys.master_files as mf

on mf.database_id = divfs.database_id

and mf.file_id = divfs.file_id

Gail Shaw SSC Guru Points: 1004504 More actions · Answer 5

It's since the last time SQL Server started.

Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability

We walk in the dark places no others will enter
We stand on the bridge and no one may pass

curious_sqldba SSC-Dedicated Points: 36502 More actions · Answer 6

GilaMonster (2/15/2010)
It's since the last time SQL Server started.

I am in a similar situation. If this is since last server reboot why did number of reads or io_stall_read went down? Shouldn't it be cumulative?

sys.dm_io_virtual_file_stats

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