(replying to both Go Canes (2 posts) and Patrick)
On 9/17/2025 1:27 PM, Go Canes wrote:
On Wed, Sep 17, 2025 at 2:48 PM home user via users
<[email protected]> wrote:
Is there a 1-to-1 correspondence between "drive" and "volume". In other
words, each drive corresponds to one and only volume, and each volume
corresponds to one and only one drive?
No, there is not. A "volume" is a logical entity, a "drive" is a
more-or-less[1] physical entity. It also depends on context[2].
A drive can be directly formatted with a file system at which time it
would be considered a volume.
A drive can be partitioned. A partition can be formatted with a file
system at which time it would be considered a volume. Sometimes the
partition itself is referred to as a volume.
One or more drives and/or partitions can be formatted as an LVM
Physical Volume (PV). Within LVM you would have Volume Groups (VGs),
and within the VGs you would have Logical Volumes (LVs). Each LV
would be a volume.
Other volume managers such as btrfs and zfs have similar concepts to LVM.
A RAID set is a volume typically created from multiple drives, but it
could also be constructed from multiple partitions. The RAID volume
is presented to the system as a drive.
So a drive can have one or more volumes, and a single volume can exist
across one or more drives.
[1] I say more-or-less because of the RAID case, and also because a
"drive" could also be a virtual disk for a VM.
[2] RAID provides a good example - from the RAID context multiple
drives/partitions are used to create a volume; from the OS context the
RAID volume is a drive.
On 9/17/2025 1:33 PM, Go Canes wrote:
> On Wed, Sep 17, 2025 at 2:48 PM home user via users
> <[email protected]> wrote:
>> A drive is divided into sectors, which are divided into blocks. Am
I right?
>
> Correct, although you usually don't care about sectors. In fact, most
> modern spinning disk drives *lie* about the number of sectors and
> their size. Only the block size and number of blocks is normally
> relevant.
>
>> For the above, does it matter whether the drive is spinning or solid
state?
>
> No, the OS hides these details (assuming it matters at all). In
> addition to NVMe SSDs you can also get SATA SSDs, and these show up as
> the usual /dev/sda, sdb, etc.
On 9/17/2025 3:17 PM, Patrick O'Callaghan wrote:
> On Wed, 2025-09-17 at 12:48 -0600, home user via users wrote:
>> Is there a 1-to-1 correspondence between "drive" and "volume". In
other
>> words, each drive corresponds to one and only volume, and each volume
>> corresponds to one and only one drive?
>>
>
> No. A drive is a physical (hardware) object. A volume is a logical
> construction. A drive may contain several volumes, and in some cases a
> (logical) volume may occupy (parts of) more than one drive.
>
>> A drive is divided into sectors, which are divided into blocks. Am
I right?
>
> No. Drives are traditionally accessed by sector, i.e. the smallest
> accessible unit on a drive is the sector, as determined by the
> hardware. It's very often 512 bytes long. A block is a logical unit of
> a filesystem, defined in terms of an integral number of sectors (both
> sectors and blocks are in practice always sized in powers of 2 so this
> works). The original versions of UNIX had blocks of 1 sector each, so
> the terminology could sometimes be mixed up, but these days most
> filesystems have blocks of multiple sectors. Although it's risky to
> generalise, it's usually safe to assume that file space allocations on
> the physical drive is in terms of blocks consisting of contiguous sets
> of sectors for faster I/O operations.
(sigh)
The day seems to be rapidly approaching when it takes more than a Ph.D.
to be a good systems administrator. I don't even have one course.
I was trying to think of a word for the opposite(?) complement(?) of
"physical" in this context. Your use of the term "logical" satisfies
that! Thank-you.
Oh did I have "volume", "block", and "sector" wrong. I sit corrected.
Thank-you.
--
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