DCache

Introduction

dCache is filesystem designed for storing large amounts of data. The data is stored amongst many server nodes, accessed through a single virtual filesystem by a variety of methods. The dCache filesystem used on stoomboot can be accessed via on nfs mount point from any of the stoomboot interactive or worker nodes. All file stored in dCache can be found under the mount point /dcache.

Usage

dCache can be used pretty much as a normal file system. The main difference is that you can not change the contents of a file stored in dCache once the file is written. So for example you can't overwrite a file:

[andrewp@stbc-i1 /dcache/atlas/andrewp]$ echo 123 > test.file
[andrewp@stbc-i1 /dcache/atlas/andrewp]$ echo 1234 > test.file
-bash: test.file: Permission denied

or append to a file:

[andrewp@stbc-i2 /dcache/atlas/andrewp]$ echo 123 > test.file
[andrewp@stbc-i2 /dcache/atlas/andrewp]$ echo 4 >> test.file
-bash: test.file: Permission denied

and when deleting a file that file is always treated as being write protected (even if the writable bit is set):

[andrewp@stbc-i2 /dcache/atlas/andrewp]$ ll test.file
-rw-r--r--. 1 andrewp datagrid 4 May  4 09:57 test.file
[andrewp@stbc-i2 /dcache/atlas/andrewp]$ rm test.file
rm: remove write-protected regular file `test.file'? y

These limitations mean that dCache is not suited for such things as compiling code on, storing large files being actively edited (e.g. word documents) or storing files under a version control system.