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Move discussion of external volumes to architectural considerations.

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Nathaniel Beaver
2015-09-11 12:28:39 -05:00
parent 98ad942bcf
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README.rst
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@@ -516,142 +516,6 @@ For example, the Linux port of the `Unity engine`_ has `issues with case-sensiti
https://archive.org/stream/Intro_to_CPM_Feat_and_Facilities/Intro_to_CPM_Feat_and_Facilities_djvu.txt https://archive.org/stream/Intro_to_CPM_Feat_and_Facilities/Intro_to_CPM_Feat_and_Facilities_djvu.txt
-----------------------------------------
Limitations on access to external volumes
-----------------------------------------
When accessing external volumes such as flash drives,
Windows assigns different capital letters to each volume
each corresponding to a different absolute path root.
This is necessary for backwards compatibility with MS-DOS,
but it is not without drawbacks.
Perhaps the most obvious problem
is that there are only 26 letters in the English alphabet.
But what does this mean in practice?
For example, the drive letter may be different when a drive is reconnected,
but applications that track recently used files
will look for files under the old drive letter,
and be unable to find the files.
I have a problem with Word when working with documents on my flash drive.
If I insert the drive days later and try to use the recently used file
list, Word sometimes says it can’t find the document.
I’ve worked out that when I insert the flash drive it’s not always using
the same drive letter – it’s F or G drive but occasionally even later in
the alphabet.
How can I change the flash drive letter or, even better, make it appear as
the same drive letter each time?
http://new.office-watch.com/2008/make-a-consistent-drive-letter-or-path-to-a-removable-drive/
Both of these problems can be solved using NTFS mount points,
but Windows doesn't use them by default.
There are also other limitations;
for example, the recycle bin doesn't work as expected.
The problem is the recycle bin. This "undo" option is maintained with a hidden
system file that is on the partition that holds the files being deleted.
Unfortuantely, when the command to delete a folder is given, the system
attempts to delete the folder using the mount point folder's Master File Table,
and not the subfolder's Master File Table. The mount point folder's MFT
doesn't host the record, and an access denied message is kicked back to you for
having the temerity to try and recycle a directory which apparently doesn't
even exist! The only solution for this is to not recycle subfolders and
directories, but to outright delete them.
http://getyouriton.blogspot.com/2009/08/serious-gotchas-with-mounted-drives-or.html
This is related to an inconsistency
of the Windows operating system:
the NTFS filesystem has a root directory,
but Windows itself has no unique root directory.
(*My Computer* roughly corresponds to a root directory in concept,
and looks like a folder when viewed in Windows Explorer,
but there is no actual *My Computer* folder anywhere on the filesystem.)
Unix, on the other hand,
has a unique root directory
and mounts drives (including removable media)_
as directories on the filesystem. [#disk_location]_
On Linux, flash drives are mounted under ``/media/``,
are assigned a directory based on their label,
and the assigned directory won't change unless the partition label changes
or the drive is manually mounted somewhere else.
For graphical file managers,
each flash drive has its own trash folders,
one per user.
https://superuser.com/questions/169980/what-is-trash-and-trash-1000
https://unix.stackexchange.com/questions/93960/why-is-linuxs-filesystem-designed-as-a-single-directory-tree
.. http://www.tmsbackup.com/cms/index.php?id=652
.. http://www.techrepublic.com/blog/the-enterprise-cloud/use-mount-points-if-you-run-out-of-windows-drive-letters/
.. https://stackoverflow.com/questions/4652545/windows-what-happens-if-i-finish-drive-letters-they-are-26
.. https://technet.microsoft.com/en-us/library/cc938934.aspx
.. https://serverfault.com/questions/83165/mount-drive-with-two-drive-letters-instead-of-one
.. https://support.microsoft.com/en-us/kb/307889
http://www.zdnet.com/article/dear-microsoft-its-time-to-stop-using-drive-letters-and-whacks/
.. https://support.microsoft.com/en-us/kb/947021
Volume mount points are robust against system changes that occur when devices
are added or removed from a computer.
https://technet.microsoft.com/en-us/library/Cc938934.aspx
.. [#disk_location]
Multics, the predecessor to Unix,
appears to be the first operating system with a root directory
and a hierarchical filesystem underneath it.
.. TODO: source
However, the motivations for such a scheme go back further.
One of the most influential time-sharing systems,
CTSS, recognized the need for accessing files independent of their disk location.
All files kept on the disk (and drum) are known to the
user only by name: the supervisor disk control module keeps
for each user a directory of names and corresponding track
locations on the disk.
https://archive.org/stream/bitsavers_mitctssMAC5_3662592/MAC-TR-16_CTSStecNote_Mar65_djvu.txt
It is desirable, from the point of view both of programming and
of disk administration, that the user have no notion of the absolute
location where his files of information are stored in the disk. Rather,
the user will refer to his files only by symbolic names and logical mode
number.
https://archive.org/stream/bitsavers_mitctssCTS_3840198/CTSS_ProgrammersGuide_djvu.txt
Unix was developed on relatively small disk drives,
so it was useful to be able mount drives anywhere on the filesystem.
You know how Ken Thompson and Dennis Ritchie created Unix on a PDP-7 in 1969?
Well around 1971 they upgraded to a PDP-11 with a pair of RK05 disk packs (1.5
megabytes each) for storage.
When the operating system grew too big to fit on the first RK05 disk pack (their
root filesystem) they let it leak into the second one, which is where all the
user home directories lived (which is why the mount was called /usr). They
replicated all the OS directories under there (/bin, /sbin, /lib, /tmp...) and
wrote files to those new directories because their original disk was out of
space. When they got a third disk, they mounted it on /home and relocated all
the user directories to there so the OS could consume all the space on both
disks and grow to THREE WHOLE MEGABYTES (ooooh!).
http://lists.busybox.net/pipermail/busybox/2010-December/074114.html
--------------------- ---------------------
Filename restrictions Filename restrictions
@@ -896,6 +760,142 @@ These are limitations of the Windows platform
which are not intrinsic to the operationg system, which are not intrinsic to the operationg system,
but nevertheless cause problems. but nevertheless cause problems.
-----------------------------------------
Limitations on access to external volumes
-----------------------------------------
When accessing external volumes such as flash drives,
Windows assigns different capital letters to each volume
each corresponding to a different absolute path root.
This is necessary for backwards compatibility with MS-DOS,
but it is not without drawbacks.
Perhaps the most obvious problem
is that there are only 26 letters in the English alphabet.
But what does this mean in practice?
For example, the drive letter may be different when a drive is reconnected,
but applications that track recently used files
will look for files under the old drive letter,
and be unable to find the files.
I have a problem with Word when working with documents on my flash drive.
If I insert the drive days later and try to use the recently used file
list, Word sometimes says it can’t find the document.
I’ve worked out that when I insert the flash drive it’s not always using
the same drive letter – it’s F or G drive but occasionally even later in
the alphabet.
How can I change the flash drive letter or, even better, make it appear as
the same drive letter each time?
http://new.office-watch.com/2008/make-a-consistent-drive-letter-or-path-to-a-removable-drive/
Both of these problems can be solved using NTFS mount points,
but Windows doesn't use them by default.
There are also other limitations;
for example, the recycle bin doesn't work as expected.
The problem is the recycle bin. This "undo" option is maintained with a hidden
system file that is on the partition that holds the files being deleted.
Unfortuantely, when the command to delete a folder is given, the system
attempts to delete the folder using the mount point folder's Master File Table,
and not the subfolder's Master File Table. The mount point folder's MFT
doesn't host the record, and an access denied message is kicked back to you for
having the temerity to try and recycle a directory which apparently doesn't
even exist! The only solution for this is to not recycle subfolders and
directories, but to outright delete them.
http://getyouriton.blogspot.com/2009/08/serious-gotchas-with-mounted-drives-or.html
This is related to an inconsistency
of the Windows operating system:
the NTFS filesystem has a root directory,
but Windows itself has no unique root directory.
(*My Computer* roughly corresponds to a root directory in concept,
and looks like a folder when viewed in Windows Explorer,
but there is no actual *My Computer* folder anywhere on the filesystem.)
Unix, on the other hand,
has a unique root directory
and mounts drives (including removable media)_
as directories on the filesystem. [#disk_location]_
On Linux, flash drives are mounted under ``/media/``,
are assigned a directory based on their label,
and the assigned directory won't change unless the partition label changes
or the drive is manually mounted somewhere else.
For graphical file managers,
each flash drive has its own trash folders,
one per user.
https://superuser.com/questions/169980/what-is-trash-and-trash-1000
https://unix.stackexchange.com/questions/93960/why-is-linuxs-filesystem-designed-as-a-single-directory-tree
.. http://www.tmsbackup.com/cms/index.php?id=652
.. http://www.techrepublic.com/blog/the-enterprise-cloud/use-mount-points-if-you-run-out-of-windows-drive-letters/
.. https://stackoverflow.com/questions/4652545/windows-what-happens-if-i-finish-drive-letters-they-are-26
.. https://technet.microsoft.com/en-us/library/cc938934.aspx
.. https://serverfault.com/questions/83165/mount-drive-with-two-drive-letters-instead-of-one
.. https://support.microsoft.com/en-us/kb/307889
http://www.zdnet.com/article/dear-microsoft-its-time-to-stop-using-drive-letters-and-whacks/
.. https://support.microsoft.com/en-us/kb/947021
Volume mount points are robust against system changes that occur when devices
are added or removed from a computer.
https://technet.microsoft.com/en-us/library/Cc938934.aspx
.. [#disk_location]
Multics, the predecessor to Unix,
appears to be the first operating system with a root directory
and a hierarchical filesystem underneath it.
.. TODO: source
However, the motivations for such a scheme go back further.
One of the most influential time-sharing systems,
CTSS, recognized the need for accessing files independent of their disk location.
All files kept on the disk (and drum) are known to the
user only by name: the supervisor disk control module keeps
for each user a directory of names and corresponding track
locations on the disk.
https://archive.org/stream/bitsavers_mitctssMAC5_3662592/MAC-TR-16_CTSStecNote_Mar65_djvu.txt
It is desirable, from the point of view both of programming and
of disk administration, that the user have no notion of the absolute
location where his files of information are stored in the disk. Rather,
the user will refer to his files only by symbolic names and logical mode
number.
https://archive.org/stream/bitsavers_mitctssCTS_3840198/CTSS_ProgrammersGuide_djvu.txt
Unix was developed on relatively small disk drives,
so it was useful to be able mount drives anywhere on the filesystem.
You know how Ken Thompson and Dennis Ritchie created Unix on a PDP-7 in 1969?
Well around 1971 they upgraded to a PDP-11 with a pair of RK05 disk packs (1.5
megabytes each) for storage.
When the operating system grew too big to fit on the first RK05 disk pack (their
root filesystem) they let it leak into the second one, which is where all the
user home directories lived (which is why the mount was called /usr). They
replicated all the OS directories under there (/bin, /sbin, /lib, /tmp...) and
wrote files to those new directories because their original disk was out of
space. When they got a third disk, they mounted it on /home and relocated all
the user directories to there so the OS could consume all the space on both
disks and grow to THREE WHOLE MEGABYTES (ooooh!).
http://lists.busybox.net/pipermail/busybox/2010-December/074114.html
-------------------------------- --------------------------------
Limited default debugging tools. Limited default debugging tools.
-------------------------------- --------------------------------