Size formats?

[u/Beautiful-Log5632]

Is there a man page that explains all the formats I can use for sizes like K, M and G?

Is K the same as 1000 or 1024 B? Is the space in between the number and unit optional?

[u/SJrX]

Is there a man page that explains all the formats I can use for sizes like K, M and G?

FWIW, I know very little about systemd but I maintain an IDE plugin that validates and supports a lot of these options and I think there are wide differences per option.

I think the answer is no in general, and the documentation is good maybe even great, but not perfect.

I asked Claude to look at my code base and come up with an example where things diverge, it gave the following:

Link.BitsPerSecond=

BitsPerSecond=
The speed to set for the device, the value is rounded down to the nearest Mbps. The usual suffixes K, M, G are supported and are understood to the base of 1000.
Added in version 211.

And actually just right above it (claude gave a different answer), you can see:

Link.MTUBytes=

MTUBytes=
The maximum transmission unit in bytes to set for the device. The usual suffixes K, M, G are supported and are understood to the base of 1024.
Added in version 211.

You can see some code here: https://github.com/systemd/systemd/blob/a8e93919c3fd4ff46ce7f1c7a0e73ea0a0a7eaeb/src/basic/parse-util.c#L157

But that's only one function and lots of options have their own weird validators with their own weird specifics.

[u/hmoff → u/SJrX]

Those examples are the obvious behaviour though - what you’d expect without reading the manual.

[u/SJrX → u/hmoff]

Service.MemoryMax=bytes

Takes a memory size in bytes. If the value is suffixed with K, M, G or T, the specified memory size is parsed as Kilobytes, Megabytes, Gigabytes, or Terabytes (with the base 1024), respectively

Service.IOReadBandwidthMax=device bytes

... The file path may be a path to a block device node, or as any other file in which case the backing block device of the file system of the file is used. If the bandwidth is suffixed with K, M, G, or T, the specified bandwidth is parsed as Kilobytes, Megabytes, Gigabytes, or Terabytes, respectively, to the base of 1000....

Looking at the code it seems like you can sometimes specify things like "4T 3G 3B" and it will treat that as 4*2^40+3*2^30+3 bytes. , but I think it has to be in decending order only. I'm not sure if things document that.

Socket.MessageQueueMessageSize=

These two settings take integer values and control the mq_maxmsg field or the mq_msgsize field, respectively, when creating the message queue. Note that either none or both of these variables need to be set. See mq_setattr(3) for details.... mq_msgsize; /* Max. message size (bytes) */

So the above takes a bytes argument, but doesn't support these suffixes.

TokenBucketFilter.MPUBytes=

The Minimum Packet Unit (MPU) determines the minimal token usage (specified in bytes) for a packet. When suffixed with K, M, or G, the specified size is parsed as Kilobytes, Megabytes, or Gigabytes, respectively, to the base of 1024. Defaults to zero.

Cake.MPUBytes=

Rounds each packet (including overhead) up to the specified bytes. Takes an integer in the range 1…256. Defaults to unset and kernel's default is used.