# RFC: VFD SWMR (virtual file driver single writer/multiple reader)

**URL:** <https://forum.hdfgroup.org/t/rfc-vfd-swmr-virtual-file-driver-single-writer-multiple-reader/6325>\
**Category:** News and Announcements\
**Created:** [October 28, 2019, 6:22pm UTC](https://forum.hdfgroup.org/t/rfc-vfd-swmr-virtual-file-driver-single-writer-multiple-reader/6325 "2019-10-28T18:22:47Z")\
**Posts on this page:** 1\
**Page:** 1

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**Author:** ![info1](https://forum.hdfgroup.org/user_avatar/forum.hdfgroup.org/info1/32/1036_2.png) [@info1](https://forum.hdfgroup.org/u/info1)\
**Post date:** [October 28, 2019, 6:22pm UTC](https://forum.hdfgroup.org/t/rfc-vfd-swmr-virtual-file-driver-single-writer-multiple-reader/6325/1 "2019-10-28T18:22:47Z")

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Hi all,

The VFD SWMR RFC has been posted on our [support portal site](https://portal.hdfgroup.org/display/HDF5/VFD+SWMR) (and here as well, for your convenience.)

[VFD\_SWMR\_RFC\_191027.pdf](https://hdf-discourse-1.s3.amazonaws.com/original/2X/e/e034d08724f1395f058c4d995c13cad86f1f0392.pdf) (1.5 MB)

> The purpose of the SWMR (Single Writer Multiple Reader) feature is to allow a second  
> process to read a HDF5 file while data is being written to it. Use cases range from  
> monitoring data collection and/or steering experiments in progress to financial  
> applications.
> 
> The existing SWMR implementation touches most parts of the HDF5 library, and  
> therefore presents significant maintenance issues. Further, it offers no guarantees on  
> the maximum time from write to read – which makes it problematic for some  
> applications.
> 
> The primary impetus behind VFD SWMR is to implement SWMR in a more modular  
> fashion – thus minimizing maintenance costs. Fringe benefits include allowing the  
> HDF5 library to make guarantees for the maximum time from write to availability of  
> data for read (subject to the performance limits of the underlying file system, and  
> presuming that the writer calls the HDF5 library frequently), and the possibility of  
> extending SWMR to NFS and object stores.

This feature was briefly demonstrated during our October 25th 2019 webinar, and can be seen [at this point in the webinar recording](https://youtu.be/_7ot4ycw3P8?t=437). We welcome your comments on this here in the forum, or if you prefer to email privately, you can send those to Elena by email at epourmal -at- [hdfgroup.org](http://hdfgroup.org).

Thank you,

The HDF Group