What happens when Linux goes out of letters for drives?

Question

Let’s say I have 50 USB flash drives.

I assume they get to be /dev/sda to /dev/sdz. What comes after /dev/sdz?

Extension of the same question - what happens with virtual xvd devices like `/dev/xvda` ? NVME devices of the format `/dev/nvme1n1` or with partiiton `nvme2n1p3` Does the old format IDE driver giving `/dev/hda` device nodes have the same limits ? — Criggie, Apr 01 '22 at 23:05
Amusingly, I once tried to help a Windows user who *had* run out of drive letters. This was years ago. He had two floppies, several compressed drives (each uses two drive letters), external drives and several CD-ROMs and DVDs. Drive A: to drive Z: is all you get. — Wastrel, Apr 02 '22 at 14:17
@Wastrel At that point you start mounting additional volumes under directories. Yes, that is a very *nix-like approach. Works fine. The underlying physical device paths are numerical, so it'll happily keep incrementing forever (`\Device\HardDisk1`, etc.). — Bob, Apr 03 '22 at 15:17

jesse_b · Accepted Answer · 2022-04-03T10:39:13.877

60

It will go to /dev/sdaa, /dev/sdab, /dev/sdac, etc.

Here is a comment from the source code:

/**
 *  sd_format_disk_name - format disk name
 *  @prefix: name prefix - ie. "sd" for SCSI disks
 *  @index: index of the disk to format name for
 *  @buf: output buffer
 *  @buflen: length of the output buffer
 *
 *  SCSI disk names starts at sda.  The 26th device is sdz and the
 *  27th is sdaa.  The last one for two lettered suffix is sdzz
 *  which is followed by sdaaa.
 *
 *  This is basically 26 base counting with one extra 'nil' entry
 *  at the beginning from the second digit on and can be
 *  determined using similar method as 26 base conversion with the
 *  index shifted -1 after each digit is computed.
 *
 *  CONTEXT:
 *  Don't care.
 *
 *  RETURNS:
 *  0 on success, -errno on failure.
 */

https://github.com/torvalds/linux/blob/master/drivers/scsi/sd.c#L3303-L3324

edited Apr 03 '22 at 10:39

answered Mar 31 '22 at 14:46

jesse_b

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I wonder how many SCSI devices one would need to attach to reach [`/dev/sdeignfully`](https://www.collinsdictionary.com/us/dictionary/english/sdeignfully)? – Tom Apr 01 '22 at 01:49
@Tom Given that it's a British English word perhaps it originated from [The Grauniad](http://grauniad.co.uk)? – Alex Hajnal Apr 01 '22 at 03:35
7

@Tom you'd have to connect 2061986601 devices (https://tenderleo.gitbooks.io/leetcode-solutions-/content/GoogleEasy/171.html) – alexyorke Apr 01 '22 at 06:55
3

@Tom why sdeignfully? – user253751 Apr 01 '22 at 09:40
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@user253751, one presumes because that's the first English word beginning `sd` that contains "full" within it. – Toby Speight Apr 01 '22 at 10:54
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@user253751 Longest word in my en-US wordlist (~275k entries) that begins with "sd". – Tom Apr 01 '22 at 17:16
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https://www.collinsdictionary.com/dictionary/english/sdeignfully says "Obsolete word for `distainfully` " – Criggie Apr 01 '22 at 23:01
... provided the `int buflen` is big enough – Hagen von Eitzen Apr 02 '22 at 06:42
1

The comment is not so long. It would be better to quote it, in case that link breaks. Though nothing wrong with keeping the link, of course. – Faheem Mitha Apr 03 '22 at 07:25

What happens when Linux goes out of letters for drives?

1 Answers1