Antw: LMDB on Ramdisk (tmpfs)

["Ulrich Windl" <Ulrich.Windl@rz.uni-regensburg.de>]

>>> Luca Foppiano <luca.foppiano@inria.fr> schrieb am 21.03.2018 um 18:26 in
Nachricht <0453D312-E481-4F93-BF31-7A5BFF55E73C@inria.fr>:
> Dear everybody, 
>     I'm using one of your component (LMDB) via a java JNDI bindings
> implementation (https://github.com/deephacks/lmdbjni 
> <https://github.com/deephacks/lmdbjni>) and I'm having an issue
> when I deploy my LMDB file on a tempfs filesystem in RAM. 
> 
> The issue do not occur when the LMDB files are stored on a "normal" 
> filesystem.
> When the data is in the tempfs ramdisk all the allocated memory ends up 
> being in
> the Dirty area (it has not been written back to the Filesytem). 

So where do you expect a dirty buffer for a RAM filesystemn to be written? To RAM? Then obviously copying RAM to RAM is just a waste of time. My guess is that ist working as designed.

> 
> Here an example using the ramdisk: 
> 
> 7ce320000000-7cfc20000000 r--s 00000000 00:26 2459                      
> /ramfs/nerd/data/db/db-en/entityEmbeddings/data.mdb
> Size:           104857600 kB
> Rss:             1255680 kB
> Pss:             1255680 kB
> Shared_Clean:          0 kB
> Shared_Dirty:          0 kB
> Private_Clean:         0 kB
> Private_Dirty:   1255680 kB    <---
> Referenced:      1255680 kB
> Anonymous:             0 kB
> AnonHugePages:         0 kB
> Shared_Hugetlb:        0 kB
> Private_Hugetlb:       0 kB
> Swap:                  0 kB
> SwapPss:               0 kB
> KernelPageSize:        4 kB
> MMUPageSize:           4 kB
> Locked:                0 kB
> VmFlags: rd sh mr mw me ms sd 
> 
> and here an example without: 
> 
> 7ca4fc000000-7cbdfc000000 r--s 00000000 fd:00 11154951                  
> /data/workspace/shared/nerd-data/db/db-en/entityEmbeddings/data.mdb
> Size:           104857600 kB
> Rss:              838124 kB
> Pss:              838124 kB
> Shared_Clean:          0 kB
> Shared_Dirty:          0 kB
> Private_Clean:    838124 kB       <----
> Private_Dirty:         0 kB
> Referenced:       764872 kB
> Anonymous:             0 kB
> AnonHugePages:         0 kB
> ShmemPmdMapped:        0 kB
> Shared_Hugetlb:        0 kB
> Private_Hugetlb:       0 kB
> Swap:                  0 kB
> SwapPss:               0 kB
> KernelPageSize:        4 kB
> MMUPageSize:           4 kB
> Locked:                0 kB
> VmFlags: rd sh mr mw me ms sd 
> 
> 
> According to my understanding the memory is dirty when 1)there are open
> transactions, 2) the data has not been written back to the filesystem 
> 
> What I don't understand is why there is a difference between filesystem and
> ramdisk? 
> Is there any reason? The application (listed above) is not writing on the 
> lmdb,
> but just reading (using reading transaction). 
> 
> Thank you
> Luca