From: Largent, Aaron (ALargent@concordefs.com)
Date: Wed May 07 2003 - 15:57:41 EDT
The Label is at the beginning of the DISK, not the
beginning of each SLICE.
The Boot Block and Superblock reside on Sector 0
and Sector 16 of each SLICE. I meant to indicate
that these measurements were relative to the beginning
of the slice, NOT to the beginning of the disk.
thanks to Darren Dunham =)
> -----Original Message-----
> From: Largent, Aaron [mailto:ALargent@concordefs.com]
> Sent: Wednesday, May 07, 2003 3:23 PM
> To: sunmanagers@sunmanagers.org
> Subject: SUMMARY: reference to really low level filesystem docs?
>
>
> I have found enough information at this point to summarize,
> but my search for more hasn't ended. I think a topical
> breakdown best fits what I've found. Please notify me of
> any horrific errors.
>
>
> ------
> LAYOUT OF A SINGLE DISK SLICE WITH UFS:
>
> Label
> -at beginning of disk, not on any slice
> -is 1 sector (512 bytes) in length
> -contains the disk name, geometry, and partition table
> Boot Block
> -located at Cylinder 0, Track 0, Sector 0
> -16 sectors in length (8K)
> -holds the boot program
> Superblock
> -located at Cylinder 0, Track 0, Sector 16
> -16 sectors in length (8K)
> -contains information about filesystem:
> -various offsets, block size, frag size, number of
> data blocks, number of cylinders, last time written,
> -also contains tunables
> -minfree, fs_rps, fs_maxbpg, optimization preference, etc
> Cylinder Groups
> -each cylinder group has backup Superblock at beginning
> -then Inodes
> -then Data blocks
>
> ------
> INODE DETAILS
>
> An Inode is a 128-byte structure that contains all details
> about a file except its name (this includes things like
> permissions, atime, mtime, ctime, owner, etc).
>
> Within the inode are 12 direct data pointers, locating the
> first 12 data blocks of the file (about 96 kB of data,
> if using 8kB data blocks size).
>
> The 13th pointer locates an "indirect" block that is nothing
> but a large array of pointers to the next 2048 data blocks.
> (about 1.6 MB of data, if using 8kB data block size)
>
> The 14th pointer locates another indirect block that contains
> 2048 pointers, each pointing to ANOTHER indirect block.
> So 2048*2048*8 is about equal to 34 GB, which is already above
> the addressable range if the fragment size is 1024 bytes.
>
> You can see most of the details of an inode by using the various
> stat() system calls, or you can cheat by using truss like this:
>
> # truss -v all -t lstat ls -la /etc/passwd
>
>
> ------
> VIEWING CONTENTS OF SUPERBLOCK:
>
> "fstyp" is the most useful tool I have found to do this
>
>
> ------
> VIEWING A FILE'S LAYOUT ON THE FILESYSTEM
>
> "filestat" is available on the internet, but Solaris
> doesn't coem with a tool that does this for you. You
> can read directly from the disk and do this, but I haven't
> figured this part out yet. "filestat" seems useful enough
> for the curious admin.
>
>
> ------
> UFS LAYOUT ALGORITHM
>
> This was perhaps the most surprising information I found
> in my search. As it turns out, the default layout for a
> file's data blocks is quite discontiguous if it is larger
> in size than about 3 MB, and the filesystem is of respectable
> size -- say 1GB+.
>
> Assuming a basically empty filesystem, the first 12 data blocks
> will be located in the first free cylinder group. The next 2048
> data blocks will be located in the following cylinder group,
> and this will continue ad nauseam.
>
> The parameters that can affect this are fs_maxbpg, optimization
> preference of the filesystem, and block/fragment size.
>
> By thinking about what kinds of data reside on a given filesystem,
> the above parameters can be tuned especially for higher
> performance of that filesystem. This may also affect how
> one would choose to mix applications on filesystems.
>
>
> ------
> INTERESTING MATERIAL
>
> manpages
> inode (4)
> statvfs (2)
> fs_ufs (4)
> dir_ufs (4)
>
> commands
> fstyp
> filestat (external utility)
> tunefs
> mkfs
> ff
>
> header files
> /usr/include/sys/fs/ufs_fs.h
> /usr/include/sys/fs/ufs_inode.h
> /usr/include/sys/fs/ufs_fsdir.h
>
> books
> Solaris 7 Performance Administration Tools
> Frank Cervone
> Solaris Internals
> Mauro & McDougall
>
> ------
> VxFS vs UFS
>
> The main differences I have found between VxFS and UFS
> so far have been logging and layout startegy. Where logging
> is an option on UFS, it is a main feature of VxFS, meaning
> VxFS check/recovery times are on the order of seconds for
> large filesystems.
>
> The layout strategy is fundamentally different for VxFS, though.
> As an extent-based filesystem, it allocates contiguous extents
> of space and then just keeps track of the offset and length.
> For large files on fairly unfragmented filesystems, this should
> result in far less overhead than looking up what could be millions
> of pointers in the comparable UFS filesystem.
>
> As a general rule, however, VxFS is prone to fragmentation,
> where UFS is not.
>
> ------
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