How do I create a large file (>2GB)

[Anthony deRito]

I want to test the largefile option on my VxFS filesystem. How can I quickly create a large file >2GB?

[Trond Haugen · Accepted solution]

dd if=/stand/vmunix of=/sparse bs=1024k seek=3000
This creates a 3 GB sparse file. IO error = no largefile capability. No IO error = normal completion. ll() file = 3gb file

Regards,
Trond

[Pete Randall]

prealloc filename 2100000000


Pete

[Jeff Schussele]

Hi Anthony,

Use the prealloc command to create a file of any size.
Since 2GB is 1024 * 1024 * 2 = 2,097,152 then it has to be larger - so

prealloc large_filename 2098000 would suffice

Rgds,
Jeff

[Rodney Hills]

You can use "dd".

dd if=/dev/zero of=/yourfile count=20

This will create a file 20*512 bytes.

HTH

-- Rod Hills

[Pete Randall]

Actually, I got kind of carried away with my zeroes there. Something like

prealloc filename 2100000

would be more appropriate!


Pete

[Jeff Schussele]

Ooops - left off a few 0s - of course it should be

2098000000

Jeff

[Geoff Wild]

prealloc is the way to go:

prealloc testfile 25000000

prealloc(1) prealloc(1)

NAME
prealloc - preallocate disk storage

SYNOPSIS
prealloc name size

DESCRIPTION
prealloc preallocates at least size bytes of disk space for an
ordinary file name, creating the file if name does not already exist.
The space is allocated in an implementation-dependent fashion for fast
sequential reads and writes of the file.


Rgds...Geoff

[Jeff Schussele]

No Pete, you were right the 1st time.
My calculation *should* have been
1000 * 1024 * 1024 *2 = 2,097,152,000

Hence 2089000000 is correct & your 2100000000 would be OK.

Prealloc operates in bytes - not Kbytes

Jeff

[Pete Randall]

Thanks, Jeff, I thought I was losing it (again)!

;^)


Pete

[Anthony deRito]

You guys are awesome. Thanks for your help on this.

I also tried the dd option on my Solaris boxes (since prealloc is not available on SOlaris). It seems that Veritas VxFS filesystem has the largefile option already on by default because it is not bombing like it is on my HP boxes. Can anyone verify this? I am running Veritas 3.5.

Anthony

[Jeff Schussele]

Whether LVM or VxVM is the volume mgr, the fsadm command will tell you if largefiles is on - just run it against the mount point.

#fsadm /mnt_point

If it doesn't return largefiles - then it's not on.

HTH,
Jeff

[Geoff Wild]

Actually, on solaris, use mkfile:

Maintenance Commands mkfile(1M)

NAME
mkfile - create a file

SYNOPSIS
mkfile [ -nv ] size [k | b | m ] filename ...

DESCRIPTION
mkfile creates one or more files that are suitable for use
as NFS-mounted swap areas, or as local swap areas. When a
root user executes mkfile(), the sticky bit is set and the
file is padded with zeros by default. When non-root users
execute mkfile(), they must manually set the sticky bit
using chmod(1). The default size is in bytes, but it can be
flagged as kilobytes, blocks, or megabytes, with the k, b,
or m suffixes, respectively.

for a 4GB file:

mkfile testfile 4196M

Rgds...Geoff

[Bill Hassell]

Note that prealloc is *not* a 'fast' way to create a large file. It will take a while. Also note that VxVM (Veritas Volume Manager) and LVM know nothing about files. That is the function of mksf or newfs. The disks are carved up with volume managers but only a filesystem knows about files. Use fsadm to verify the capability of the filesystem:

fsadm -F vxfs /var
fsadm -F vxfs /dev/vg00/rlvol8

(note the use of rlvol8, not lvol8). Note also that you need to add the largefiles option in fstab. This is also a check to make sure the filesystem supports largefiles..if it doesn't then mount will fail. You can change any vxfs filesystem to add support for largefiles with fsadm.